The combine ratings do correspond to the real ratings, but the combine ratings can be wrong until you scout a player. For the 3 players that had 4.40 as their 40 time but had wildly different ratings, you probably didn't scout the two that had 93 and 95 as their Speed ratings. A 4.40 40 time equates to an 87-88 in Speed. If you scouted the other two, the 93 Speed would have dropped to roughly 4.35 and the 95 Speed would have dropped to roughly a 4.30. Here's the rough 40 times -> Speed ratings I've been able to put together.
40 time -> Speed Rating
4.40 -> 88
4.38 -> 90
4.35 -> 92-93
4.30 -> 95
4.27 -> 97
4.25 -> 98
4.23 -> 99
Bench Reps -> Strength Rating
30 -> 89-90
31 -> 91
32 -> 92-93
33 -> 94
34 -> 95-96
35 -> 98-97
36 -> 99