Run blocking really isn't the issue. If you watch how the linemen release blocks and get upfield to the next level, the timing is fine (there is a variety to when/if the blocker gets that next level in-time - which is what we want). Raising the block, even one point, will disrupt the timing as they're holding the blocks too long and ultimately will make it so good runs happen mainly when the second-level guys are out of the way.
The issue with 51 HUM tackling is that the CPU backs become more tentative because of that higher slider (which would be higher than the fumble slider). The HUM value of 50 keeps it on the same level as the CPU fumble slider, and CPU backs will thus hit the holes more directly instead of being tentative (i.e. "afraid of being hit").