There was one little sour note yesterday as the Republicans celebrated their Thursday budget victory in the House: the deficit in their spending plan may not be a little below $100 billion, as they proudly proclaimed, but about $10 billion above that magic number.

Conveniently brushed aside as the House ended months of turmoil by approving a fiscal 1983 budget with a supposed deficit of $99.3 billion was a crisp, matter-of-fact letter from the Congressional Budget Office that refigured the numbers and said the likely deficit would be $109.8 billion instead.

In its routine costing-out of the Republican-drafted budget and a subsequently defeated Democratic alternative, the nonpartisan CBO concluded that both parties had underestimated their proposed spending, the Republicans only somewhat more than the Democrats.

It mattered more to the Republicans, however, because a less-than-$100-billion deficit was a key selling point for their plan, while the Democrats made no pretense of staying under $100 billion.

Specifically, the budget office figured that the Republicans underestimated defense spend-out rates by $2.2 billion, overestimated revenues from off-shore oil leasing by $3.8 billion, undercounted interest costs by $2.5 billion and came up $2 billion short in assorted other areas.

Even though Democrats tried to make an issue of CBO's numbers, Republicans scoffed at "all kinds of figures floating around this place," as Rep. Delbert L. Latta (R-Ohio) put it, and nobody paid much attention in the widely shared relief over finally getting a budget through the House.

But now the issue may return to haunt the Republicans, especially Senate Republicans, who used assumptions and program estimates that were virtually identical to CBO's in computing their own budget, which projected a deficit of nearly $116 billion when it was approved last month.

These were also the assumptions and estimates used by both Democrats and Republicans, including White House officials, in trying to negotiate a bipartisan budget compromise earlier in the year.

The problem is that the Senate now has to go to conference with the House to resolve differences over their separate budget plans, and, somewhere along the line, the two houses have to agree on their accounting methods--presumably before getting to the really tough issues of tax and spending policies.

If the Senate sticks by the CBO's numbers, a compromise deficit would probably exceed $110 billion. This could cause real problems for House Republicans. House Minority Leader Robert H. Michel (R-Ill.) has bluntly warned that a split-the-difference deficit--even one that falls halfway between $99 billion and $116 billion--might not get through the deficit-skittish House. "We've got to do a lot better than split the difference," Michel said.

But if the Senate rejects the assumptions upon which it based its own budget in order to help the House out of its difficulties, it would be open to charges of "cooking the numbers" to make the deficit look less alarming--which is just the kind of thing that makes some of its own members go on filibustering rampages.

"Whatever we do we aren't going to jimmy the numbers," Senate Budget Committee Chairman Pete V. Domenici (R-N.M.) said yesterday. But he indicated he thought there was some leeway for compromise, and other sources said an accommodation was likely.

Democrats preferred to watch the Republican squirming in silence. But they will undoubtedly be heard from as the numbers game unfolds at the conference, which is scheduled to open Tuesday afternoon.

The Senate and House budgets do not appear to differ much in aggregate numbers, with CBO estimating actual policy differences at no more than $7 billion as a whole in Senate and House budgets of more than $750 billion.

But, according to budget committee reviews, there are politically hard-to-resolve differences over spending for Medicare and Medicaid, savings from federal retirees' cost-of-living increases, the level of cuts in domestic appropriations, latitude for supplemental appropriations for fiscal 1982, tax increases for 1984 and 1985 and language calling for unspecified but potentially big Social Security "solvency" measures.

So close and shaky was the House vote--219 to 206 on final passage--that congressional leaders say defeat of a conference report is possible, despite the new round of turmoil it would cause.

And another budget stalemate could jeopardize legislation to extend the debt ceiling and keep the government in operation after current borrowing authority runs out at the end of the month.