teams usually tack on .5 seconds to pro day 40's.
Its not just pro days, its bowl games as well.
Pretty much every player's 40 is faster at pro days, year after after year.
The NFL combine a fully automatic timing system which is more accurate and slower than hand times.
Teams might be setting up FAT systems at pro days but if they are using hand times they will be faster. You would expect on average a hand timed run to be about 0.24 seconds faster than when using FAT systems.
I'd also like to think the combine removes all bias where as there may be some type of bias or less accuracy at the pro days, as different people will be performing the measurements for each pro day, where as 1 trained team is doing all the measurements at the combine and they are doing them the exact same way.
Edit: this also brings in to some context why back in the day some players had significantly faster 40's. I don't know when they instituted using FAT at the combine but if they were using hand times it would really discredit all of the times compared to current day players