Top Carter administration officials revealed yesterday that federal government spending has fallen $7.6 billion behind scheduled in the first four months of this fiscal year and that they are "perplexed" and "baffled" by the development.

The mysterious "shortfall" from budgeted expenditures, Treasury Secretary W. Michael Blumenthal told reporters, leaves uncertain the extent of the real federal deficit for this year and next.

And "the other side of the coin," he added, is that the "shortfall" in spending might necessitate an increase in the administration's economic stimulus package.

The problem of "shortfalls" in federal spending first turned up in a significant way last year, when the Ford administration was equally baffled by what originally was estimated to be an $11 billion shortfall between March and October.

Eventually the pace of spending quickened slightly, but about $9 billion less than budgeted the fiscal 1976 was expended. Much of this was finally attributed to a slowdown in defense spending, and a part to lower interest rate costs to the government than originally estimated.

Office of Management and Budget Director Bert Lance said in a telephone interview yesterday that the OMB "has absolutely no idea" what is causing the current shortfall or whether it will continue.

A hint of expenditure-estimating trouble - but nothing to indicate its extent - was contained in President Carter's amended budget sent to Congress Feb. 22. Experience suggests there is "some general upward bias" in making outlay estimates, Carter said.

Blumenthal conceded that it may seem difficult for the public to believe that the government not only is making inaccurate estimates of how it spends its money, but can't explain the errors. "We're going through department by department on their costs," he said. "But it's easy to pose the questions, and harder to get answers."

Lance said that he had discussed the situation on Tuesday with Carter, Blumenthal and Charles L. Schultze, chairman of the Council of Economic Advisers, and that 'we are all concerned."

He said that one possibility was that spending scheduled for fiscal 1977 which ends Sept. 30, might "spill over" into fiscal 1978, a trend not welcomed by an administration trying for a steady reduction in deficits with a goal of a balanced budget by fiscal 1981.

Blumenthal stressed yesterday that the Presidents firm on his goal of a balanced budget by fiscal 1981. "He insists on a program to get us there," Blumenthal said, 'and that means we are going to reduce the deficit substantially by fiscal 1977, and move toward a balanced budget in fiscal 1981."

In each of the first three months of fiscal 1977 - October, November and December, 1976 - the shortfall was a little more than $1 billion, totaling $4.2 billion.

But in January the drop behind scheduled expenditures was $3.4 billion, 'and it's continuing," Blumenthal said. Showing his frustration, Lance added: "We may be getting to the point where (a budget director) can't spend any more money."

Blumenthal brought up the shortfall question in the course of suggesting that the currently forecast $68 billion deficit for fiscal 1977 and the $57.7 billion deficit for fiscal 1978 are inexact estimates that may prove too high.

'Unless there's a tremendous reversal (in the rest of fiscal year), the deficit is going to be smaller," Blumenthal said.

But he really acknowledged that this would be a mixed blessing, because the lower spending figure would provide less of a boost for the economy, which might have to be compensated by boosting the $31.6 billion stimulus package.

The Treasury official said that if the economy "is not picking up, and we found that is due to the shortfall, we clearly would have to reconsider" the stimulus package.