You mostly have to work deeper into analyzing what is going on yourself. There is nothing intrinsically wrong about the way OOTP10 is handling historicals. This version got a lot of testing, including historicals.
Of course, you might have stumbled upon a quirk in the game that's affecting your teams. You would have to closely examine the stats from your seasons and compare them to real life and see if you can detect a significant difference at a detailed level. That would make it easier for others to comment.
On the other hand, you could be in a major slump. It happens.
