Some schools allow their players to run in the spring for NFL Scouts. In this years NFL combine there were 21 players that ran for NFL Scouts in the spring and also ran at the NFL Combine. Only 10 of them improved their times and 11 actually got slower. All 21 players trained at a combine prep facility, but at the end of the day, fast guys run fast.
Plus as scouts and GMs will tell you by the time that they get to Indianapolis for the NFL Combine their NFL Draft Board is set. Most teams have around 180 players on their draft board so the NFL Combine is about confirmation and cross checking. At the end of the day “it is about ball” as one GM told me.