Most of the 100,000 federal/military retirees here will get a cost-of-living (COL) catch-up raise of about 4.3 percent in their May annuity checks.

Except that some retirees will get less. And some others will get a pension increase, but take a pay cut.

Confused? Welcome to the patchwork world of the federal/military retirement program.

First things first: The basic COL raise for retirees this year will equal the rise in living costs for 1982. The exact amount of the increase will be announced later this month, when living-cost data for the month of December is computed and announced.

But thanks to changes Congress made last year to take effect this year, some retirees will get more than others. And some former military personnel now working for Uncle Sam as civilians will take a pay cut each time they get a raise in their military pensions.

Previously government and military retirees got COL adjustments twice a year. The raises, effective in March and September, arrived in paychecks in April and October.

Congress eliminated the September increase two years ago. Last year it voted to delay the date of the next three COL raises by one month each year. Translation: The 1983 retiree raise will show up in May checks; the 1984 adjustment will come in June and the 1985 COL adjustment will be in July.

In addition, Congress voted to limit the increases for younger retirees--those who have not reached their 62d birthday. As a result those under-62 retirees will get a 3.3 percent raise this May. Persons retired on disability, those getting survivor annuities and anyone over 62 will get the full amount of the COL raise.

Changes were also approved for the 140,000 plus military retirees--many of them in this area--who now have civilian federal jobs. The primary one will reduce the civilian federal salary (except for persons retired with service-connected disabilities) by one dollar for each one dollar increase in retired military pay. That offset will apply to all COL raises through 1985.

Only about 10 percent of all federal retirees are under age 62, and therefore due reduced raises. But the majority of the military retirees are under age 62 and subject to the retirement income changes.