When Republicans won the House majority, some of their most conservative members pledged to use their power to slash the budgets of the federal agencies they claimed had been weaponized against them — chief among them the Federal Bureau of Investigation.

So when Speaker Mike Johnson unveiled the package of six government spending bills he had negotiated with Democrats, which President Biden signed into law on Saturday, he touted the “deep cuts” — 6 percent — Republicans had secured to the agency’s budget.

But the story of the F.B.I. cut is not so much one of how House Republicans used their slim majority to raze the budget of an agency they claim has gone rogue. Instead, it is a remarkable yarn about how a single powerful senator used budgetary sleight of hand to steer hundreds of millions of dollars to a single project in his state, only to see the money slashed by members of his own party after he retired.

Out of the $654 million lawmakers agreed to cut this year from the F.B.I.’s operating budget, $622 million came from eliminating what was essentially an old earmark: money for construction at the bureau’s campus at Redstone Arsenal in Huntsville, Ala. The funding was placed into the budget years ago by Senator Richard C. Shelby of Alabama, the legendary pork-barreling veteran who retired in 2022 at the age of 88.

The actual cut to the F.B.I.’s operating budget — mostly for personnel and operations — was roughly $32 million, or 0.3 percent.

Ultraconservative Republicans like Representative Chip Roy of Texas who voted against the spending package this week, deriding it as full of budgetary gimmicks, pointed to the elimination of Mr. Shelby’s pet project as a prime example of how little his party had actually been able to cut.