The DB glitch is due to a certain zone coverage the defense runs on 3rd and long and at the end of half/game. Most likely a prevent or deep cover 2. The safety when the AI has switched it gameplan to zone aggressive will run into "no man's land" whicjh is about 15 yards of the line of scrimmage. The CB stays in a short/medium zone and "passes off" the WR to the safety who should be covering over the top. The safety picks clover in the no man's land while the WR runs right past him on streaks especially, but flags and posts are also open.
Now pass coverage is how tightly the DB's will mirror the WR routs. the Interception slider is how quickly the DB reacts to the ball in the air. If you crank up the INT slider, the safety in no mans land may be able to start running back once the ball is in the air and may make it back if you are late on the throw, but if yopu throw the ball while the WR is still running the route, the saftey has no chance. If you throw the ball late while the reciver has completed the streak or nearing thew end, the safety may get back if the INT slider is high enough.
I can say with certainty, the lower you put the human coverage slider, the better your AI controlled DBs will cover the WR. This slider is reversed for the human. It appears to be the same for the AI, however, most of the AI DB play is controlled by the INT slider.
To answer your question, its a programming glitch. if the glitch did not exist, 50 cobverage and INT for the AI on Hesiman would work just fine. They appear to get better as you lower the coverage slider which most ancient alien theorists suggest. However, the glitch is basically a free TD regardless of sliders because the safety will go into the no man's land on 3rd and long and at the end of the half. It has nothing to do with the sliders. The only reason Varsity works is because the game is supposed to be "easier" and the AI doesn't go into zone aggressive which is apparently supposed to make it harder to complete a pass so its used on the higher levels, but instead, it blows up the game.