Franchise - QB xp non reliable

I'm doing a franchise with the Browns. I had this problem. Week 16 my qb (Sanders, 75 overall, star dev) reached 3500 yards and 30 tds. He got 23000 + 23000 XP!!!! That's crazy! He got 9 skill points at once! This is not realistic.

My options are 80% in qb experience (xp sliders) and 110% for being 23 years old. I have also Flacco as backup, which is a Mentor (I don't know how much xp increase a mentor gives).

On the "goals" page of Shedeur, reaching the goals above would have worth about 8k each, not 23k!!! 

Did anyone experience the same problem? Can anyone help me understanding how xp works? I really want a realistic experience with franchise, not a player that moves from 60 to 90 in 1 year only because he reaches 3500 yards.

Thx for any help