DAVID STOCKMAN'S position is more awkward than ever these days. He's the budget- numbers man for an administration that is simultaneously offering two sets of numbers for the same budget. That's double-entry bookkeeping in an unusual sense of the term. When he appeared before the Senate Budget Committee last week, he was welcomed with more than the customary amount of bellowing about "phony, false and dishonest" figures, as one of the Democrats put it. But it's not really a case of fraud. The explanation is a good deal more intricate--and interesting--than that.

Mr. Stockman is in a box. When Congress began to write its budget for next year, it used assumptions about future economic growth that were, even then, optimistic. The budget calculations require one set of assumptions or another, and optimism holds down the estimates of spending and the deficit. In writing the budget, Congress then did a number of things that Mr. Reagan didn't much like-- adding, for example, the tax increase. Mr. Stockman was one of the people who worked out the agreements leading to the president's notably reluctant acceptance of the congressional budget. You have probably noticed the strain already developing between the White House and Congress over it.

The last thing in the world that Mr. Stockman would want to do, at this delicate moment, is to publish a new and sharply less optimistic set of economic assumptions. That would undercut the base on which the budget figures stand, reopening all those sensitive issues between Mr. Reagan and Congress. Mr. Stockman would never have published mid-year estimates voluntarily. But it was required by law. He complied by putting out numbers similar to those on which the budget was based, although by late July they were-- to put it charitably--visibly obsolete.

At the same time, turning to the other horn of the dilemma, he and others in the administration have tried to let the public know that they haven't lost their marbles altogether. They have sought to signal, publicly, that they know the recession is here. They know that the prospects for the recovery are not promising, and that the deficit's impact on the financial markets is going to be greater than the official forecast suggests.

That's why the unnamed "senior official" at the White House, in presenting the mid-year estimates to the press the other day, went to such remarkable lengths to disavow them as a realistic forecast. That's why the secretary of commerce, Malcolm Baldrige, declared publicly last Monday that the deficit would be a good deal larger than the mid- year estimates indicated. The administration wants you to know that there are two sets of numbers, the official ones, required by the politics of a very fragile budget compromise, and those that it considers the real ones.

Presently there will be a third set, when the Congressional Budget Office publishes its own estimates. With that, the budget debate will become an exercise in triple-entry bookkeeping.