look at closer at #4 and #5. they got other skills very high(DV, HN, PS), so I think this is the clue.
Not everyone necessarily trains only respectable or 18-19 year old players. I certainly don't, so this is not much of a hint.
High secondary skills will, indeed, speed up your training. Or to put it in another way, players with comparatively lower secondary skills will train their primary skills slower.
Other than that, I am in the same boat with you -- training is a process that takes into account multiple variables, so training speed cannot always be predicted with absolute precision.
"I don't know half of you half as well as I should like; and I like less than half of you half as well as you deserve."