vgates said:
I think it's your training rotation. Most of my players are closer but I also train backs so I use:

clearances x2
tacklingx2
running x2
intercepting passesx2,
sprint x2

Intercepting passes also adds a little to speed. The sprint is just a non training day for the xDF but alters the rotation, perhaps enough to even out the training effects.

OR you just bought players with too big a difference to smooth out