I think the agency presidents may be able to speak to this better, but I think it does vary by agency. If you take a natural science or engineering researcher, you'll see that their core funding is through a program called the discovery grants program. The discovery grants program is typically on a five-year cycle. The amounts don't change; they're set for the five-year period. Someone reapplies. Typically, their money might go up somewhat or go down, depending on their research performance, and that provides core funding that they can rely on to enable their students to pursue their research.
The other things are layered on top. There are more targeted programs, of varying scales and different objectives, that can change how much a researcher gets from year to year.