The 100,000 Washington area residents who get federal retirement checks were (legally) short-changed this month when they received annuity payments that are supposed to reflect a cost-of-living adjustment (COLA) for 1982. The COLA was supposed to be either 3.3 percent (for retirees under age 62) or 3.9 percent for survivor annuitants, disabled retirees and retirees over age 62.

But it didn't work out that way. Very few people got the exact amount of the full increase due them.

They did not get the full COLA because Congress last year ordered a change in retirement computation rules effective this month. The change means that most retirees and survivors will lose from 1 cent to 99 cents a month of the full COLA due them.

Prior to the rules change, the government rounded annuity increases to the nearest dollar. In other words, if you were due a raise of $10.49 it was rounded down and your actual increase was $10. If your raise was $10.50 or more, you got an increase of $11.

Congress last year made major changes in federal retirement payments that are fully indexed to the cost of living. It said that the COLA raises were to be delayed one month a year in 1983, 1984 and 1985. That meant that the COLA due on March of this year did not go into effect until April (payable in May). Next year's raise will not reach retirees until June, and the 1985 increase will be in July checks.

Congress also decided to cut costs by giving younger retirees (those who are under 62) smaller COLAs. This year they will get a 3.3 percent raise, compared with 3.9 percent for older retirees, survivor annuitants and the disabled.

One of the seemingly minor changes it made--rounding raises to the lowest dollar--didn't get much attention at the time. But it is now.

Many retirees live on very tight budgets. Many of them watch the COLA news very closely. Many of them thought they knew, to the penny, what they were going to get in their May annuity checks.

When the rounded-off checks arrived, a lot of retirees got a jolt. That is why the telephone lines at the OPM are so busy this week.

The rounding off is nickel-and-dime stuff to most retirees. But it will amount to a $1 million dollar-a-year savings to the government, and a million dollar annual loss to retirees.