I saw how it works during my grad studies in molecular bio (in the 80s, it hasn't changed). We had a NIH or NSF grant (can't remember which- it was ~40 years ago). About 5 months before the end of the grant, our advisor told us to go on a buying spree and get anything we wanted for the lab. We would not have spent the entire grant by the end and would lose that money, Instead of rolling it over that to the next grant and ask for less next year, the next grant would get less $ (by a lot). Across the whole govt., that is mega moola.