The Carter administration disclosed yesterday that the fiscal 1978 budget deficit it thought last January was going to total $61.8 billion probably will come closer to $53 billion - but not because anyone in the government planned it that way.

The $8.8 billion disparity stems mostly from a continuation of the mysterious government spending shortfall that plagued budget markers last year. While officials thought they'd squelched the problem, it apparently is back again this year - and no one knows why.

The shortfall first appeared in mid-1976, when budget officials discovered by accident that the various departments and agencies weren't spending what they were supposed to. In fiscal 1977, the disparity reached $14.7 billion - five times that of previous years.

While the shorfall happily results in a lower federal budget deficit each year, it also poses a serious problem for economic policymakers: The reduced deficit also provides visibly less stimulus to the economy than the administration planned.

The Congressional Budget Office estimated several months ago that the shortfall in 1976 resulted in a significantly slower growth rate for the economy during theat year than otherwise might have occurred if the government had spent what it was supposed to.

Carter administration officials made a full-scale effort to eliminate the shortfall in 1977, even going to the point of compensating for underspending in its budget allocations to individual bureaus and agencies this past January. Officials thought then they had the problem licked.

Budget officials say they're "re-examining" the whole situation, but they're not at all certain they'll be able to do anything about it. At this time in 1977, officials were forecasting only a $4 billion to $5 billion shortfall. It ended up at $14.7 billion.

The $8.8 billion shortfall reported yesterday comprises $8.7 billion in underspending and $100 million in lower-than-expected tax receipts - a tolerable revenue-ectimating error in an economy this size. The figures apply to fiscal 1978, which ends in October.

The shortfall would reduce the fiscal 1978 budget deficit to $53 billion from the $61.8 billion estimated in January, and would pare the fiscal 1979 deficit slightly to $59.6 billion from the $60.6 billion projected before.

By far the bulk of the spending shortfall occurred in the military budget, which is $1.5 billion behind schedule. Next was energy spending, short by $1 billion, mostly because of changed plans for the strategic petroleum reserves.

Others include the Export-Import Bank, short $600 million; Social Services, short $600 million; and net interest, $600 million. The rest were scattered through virtually all major areas and departments. There also were adjustments in estimates of revenues from oil receipts.

The question now is what if anything to do about the reduction in spending levels. If the administration wanted to make up for the loss of stumulus, it could recommend an increase in this year's tax cut, but White House officials say that is unlikely.

Instead, President Carter most likely will settle for the reduced federal deficit, and try to work the bugs out of the system i fiscal 1979. The prospect of a lower federal deficit theoretically ought to please the financial and currency markets, although even that is not certain.

Meanwhile, the shortfall continues, inexplicable - threatening to have a growing impact on overall economic policy - and budget makers seem powerless to avert it. The figures cited in the report refer to actual outlays, and not to authority to commit funds in coming years.