Although the president listed "economic conditions" as the official reason he limited the next federal pay raise to 3 1/2 percent, the real reason is simpler. The White House just flat doesn't believe that government workers make 18 percent less than people doing the same jobs outside government. Neither do a lot of people in the private sector!

The raise will bring the average white-collar federal worker here to $31,860, and the average fed outside of Washington to $26,167.

According to data collected by the government, Uncle Sam is one of the great skinflints of all times, paying all but a handful of low-grade clerical employes a lot less than they could get on the outside.

Government employe unions say the figure is actually on the conservative side, and that in fact the gap between industry pay and government pay is a little over 20 percent.

President Carter didn't believe survey results. Nor did President Ford. Nobody in the White House in recent times has given government employes all they are entitled to based on surveys conducted by government employes.

Nobody has suggested that the Bureau of Labor Statistics is playing games with the data. The 100-year-old Labor Department agency is considered tops in its field. Few question its motives and everybody uses its numbers.

What Presidents Carter and Reagan have suggested is that the BLS survey -- set up by Congress -- is too narrow and only draws pay data from the nation's elite companies -- ignoring salaries paid in middle- and small-size firms employing nine of every 10 Americans.

Many government workers ask what is wrong with linking their pay with private industry's best? If Uncle Sam wants the best, they contend, he should compete with the big boys, and not worry about mom and pop stores.

Congress rejected a Carter plan to expand the BLS survey to include more smaller firms, and to match up salaries paid state and local government workers against their federal counterparts. It also balked at a proposal to regionalize federal pay, linking white-collar government salaries to local labor markets.

The Reagan administration plans next year -- if it gets another year -- to again ask Congress to look at the system of comparing federal and private pay. If that happens the pay "gap" could disappear, even if it really doesn't go away.