I learned one thing for certain at the 106th Kentucky Derby. When you fully expect to win $15,600 on a bet, and then you lose, the effect can be mildly depressing.

But other than that, I am not quite sure what happened at Churchill Downs on Saturday. I am not sure why the principal contenders ran as they did. Nor do I know what great handicapping lessons might be drawn from the outcome of this Derby.

Plugged Nickle, the object of my affections and of my 12-to-1 wager in the Derby future book, seemed to have impeccable credentials coming into the race. dHe finished seventh. Rockhill Native, the post-time favorite, also possessed solid virtues. He finished fifth.

Genuine Risk came to Churchill Downs with the type of preparation that almost never gets the job done in the Derby. Yet she ran brilliantly and became the second filly in history to win America's most famous race.

First, Rockhill Native and Plugged Nickle did not go dramatically off form. They ran their normal races (which were never that spectacular). But they finished behind the filly, who exploded to run an unforseenably big race, and also behind three California invaders, who generally had been underrated by us shortsighted Easterners.

Second, this Derby was heavily influenced by a factor that most of us have been able to overlook in recent years: the distance capabilities of the horses.

During the 1970s, many colts won the Derby because they were enormously superior to the opposition.Thoroughbreds like Secretariat, Seattle Slew and Spectacular Bid towered over the other horses of their generation and could have beaten them at any distancce from two furlongs to two miles.

But this kind of supremacy is something of a rarity. The relative ability of horses usually varies at different distances. This distance capability is heavily influenced by breeding.

Democratic Americans may be more reluctant to admit than genes determine destiny, but the English are more comfortable with the notion and they place a heavy emphasis on horses' inherent distance capabilities. They might see a horse win overpoweringly at 1 1/8 miles and dismiss as preposterous the notion that he could succeed at 1 1/4 miles.

Rockhill Native and Plugged Nickle usually would run slowly during the final furlong of their 1 1/8-mile vitories, and they may have been providing evidence that the Derby was going to be too far for them. I dismissed those clues at the time; now the evidence seems fairly compelling.

Even though they faded in the stretch Saturday, Plugged Nickle and Rockhill Native did not run all that terribly. For the Louisville Courier Journal's poll of writers, I consulted my speed charts and predicted that Plugged Nickle would win in 2:03 1/5. In fact, he ran in 2:03 3/5. The trouble was that six horses ran faster.

Contrary to the widespread notion that this is a very weak generation of 3-year-olds. Genuine Risk's winning time of 2:02 suggests otherwise. So does the fact that Rumbo and Jaklin Klugman finished within two-fifths of a second behind her.

People who do not understand racing often say that time is unimportant, or at least that it is relative. In fact, it is the one objective measurement of horses' abilities, and it can be analyzed in absolute terms.

A speed handicapper can study the times of other races run on each Derby day, judge from them the inherent speed of the racing surface, and then compare with some precision the times of horses who have won the Derby in different years.

I explained the process in some detail after Spectacular Bid won the Derby last year. I believe I can say with some accuracy that if each of the Derby winers since 1972 had run over Saturday's Churchill Downs track, these would have been their times:

Secretariat 2:00 1/5

Affirmed 2:01 2/5

Spectacular Bid 2:01 3/5.

Genuine Risk 2:02.

Riva Ridge 2:02.

Foolish Pleasure 2:02 1/5.

Seattle Slew 2:02 1/5.

Bold Forbes 2:02 2/5.

Cannonade 2:03 4/5.

Genuine Risk clearly is a genuine thoroughbred; she compares favorably with some of the most acclaimed horses who have won the Derby. Yet she never had run as fast as she did on Saturday. Only two weeks earlier, she had lost by 1 1/2 lengths to Plugged Nickle in the Wood Memorial Stakes at Aqueduct, and she was not gaining on him in the stretch.

Before the Wood, the filly had had only two untaxing races against members of her own sex. She did not have the necessary conditioning to beat a sharp horse like Plugged Nickle.

Afterward, though, the Wood might finally have given Genuine Risk that condition. "The race had to benefit her," trainer LeRoy Jolley argued, "and she has more room for improvement than Plugged Nickle does."

But even Jolley did not fully believe this line of reasoning (or else he would not have been so reluctant to come to Churchill Downs). And it would have been very difficult for a handicapper to anticipate that Genuine Risk was going to run a race such as she never had run before.

It is because of such unforeseeable occurrences that some people can collect $28.60 for $2, while others are left to cry over the big scores that might have been.