For the difference in value, it s du because of two parameters age and quality. If you take two player age 20 no special ability same position ( and same number of possible positions) same skill ( and SP to next skill) you may have one at 800K and the other at 2 millions. The 2 millions lad will train much faster ( and gained skill points ) than the 800k one. Although 800k may have a better rating than 2M in some game it s not connected Rating and Quality.
What I have noticed is playing with the same formation with same orders improve the rating of the players ( as if they get used to play together) and sometime a good player never have a good rating ( maybe is don t suit your team strategy and positioning) .