A SHOPPER at the Giant supermarket on Glebe Road in Arlington was confused by the warehouse prices of the 24-, 32- and 40-ounce bottles of Giant grape juice. And for good reason. All three sizes were the same price: 99 cents. The manager verified that those were the correct prices.

Such peculiarities, according to Giant headquarters, are an unintentional byproduct of the changeover from its traditional pricing system to warehouse pricing, which sets prices on a new basis. While careful shoppers have always found some inconsistencies in pricing, warehouse pricing has brought new attention to such problems.

Warehouse pricing has nothing to do with what Giant has in its warehouse. Rather, it is defined by Odonna Mathews, Giant's consumer adviser, as "competitive with the prices in a warehouse store like Basics," resulting in "lower prices than you traditionally find in a supermarket." Warehouse prices are chosen on the basis of what other warehouse-type stores are offering rather than strictly on supply and demand at the wholesale level. Unlike regular advertised sales, warehouse prices have no set time during which they will be in effect.

Thus, warehouse pricing seems to have disrupted the logic of bargain shopping. Shoppers can no longer depend on the more-is-less shopping philosophy; the products don't necessarily get cheaper the larger the container you buy. Currently at Giant, some of the smaller items are better buys than a larger size of the same item: For example, the 16-ounce size of Giant Pork & Beans is 26 cents a pound, but the 28-ounce size is 29 cents a pound. Similarly, a 16-ounce package of Giant Elbow Macaroni costs 41 cents a pound, while the 48-ounce size is 43 cents a pound.

In a survey at the Giant on Columbia Rd. NW last week, the following oddities were found in warehouse-priced items:

The 6-ounce, 12-ounce and 18-ounce cans of Hunt's Tomato Paste all had unit prices of 84 cents a pound.

The warehouse unit prices of Cheer detergent fluctuated strangely: 64 cents a pound for the 49-ounce size, 65 cents a pound for the 84-ounce size and back to 64 cents a pound for the 10-pound, 11-ounce size.

A one-pound jar of White House applesauce was selling at 32 cents a pound, and three jars for $1. That's four cents more than you would pay if buying them individually.

According to Mathews, the inconsistencies were an oversight made when the chain switched to warehouse pricing seven weeks ago. "It's an astronomical job when you have a couple thousand price changes," she said.

Gerson Barnett, senior vice president for the department responsible for setting prices, said that the changes in the problems cited above have been going out to all the stores and would be corrected by this weekend. He said that the prices would be lowered on some sizes. Mathews noted that the public has pointed out additional pricing peculiarities and that Giant has been correcting them as they are brought to its attention.

Barnett claimed that some of the price mistakes were caused by manufacturers changing the content sizes of their items.

Also, at any store, price fluctuations depend in part on "manufacturer allowances." For example, Maxwell House may be offering its smaller size jar at a better value than the larger size to one warehouse store, and Giant will try to match that price.

Smaller packages, therefore, are sometimes better values than larger. Odonna Mathews says that this is a phenomenon that's been developing for the past year or so in the marketplace, and not just at Giant. She did agree that it is occurring more now as a result of warehouse pricing.

Looking at unit prices and buying the cheapest per unit is the practice Giant suggests these days. As Barnett admitted, however, many people aren't aware of what unit pricing means -- the price of an item per pound, quart, count or square foot.

If you're concerned with economy shopping, then, a trip to the supermarket may take arithmetic skills you didn't have to use before -- and a watchful eye. Frank Sterling, the grocery manager at the Giant on Columbia Road, said that he gets five or six questions a day from shoppers who ask, "What's my better buy?"

Safeway has also incorporated warehouse pricing, but in a two-hour survey at one store, similar to the survey at the Giant store, there were few instances where the smaller size offered a better value. Ernie Moore, a spokesman for Safeway, said that the public relations office has received no calls from consumers about pricing discrepancies. He did agree, however, that the sale signs themselves sometimes were a source of confusion for customers. Warehouse pricing at Safeway has meant additional sale signs. "We have so many signs in the stores now, that some [unit prices] are getting covered, up," said Moore.

In addition, some signs have just prices written on them and don't identify the item that's on sale. Some items have three sale signs identifying them. In some cases, the sale signs aren't near the item.

Warehouse pricing has resulted in lower prices on some items, but it has indeed created confusion and made shopping for good values a complicated process. As one shopper put it, although he tries to pay attention to unit pricing and sales, "I come home from work tired, and I just want to get out of here."