Why is a player worth more than another with same age and much better stats?

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Why is the player on the left (Quality 17) worth considerably more than the player on the right (Quality 23) when they are both 18 years old?

I understand that a player that is worth more will learn faster, but the cheaper player above has much better stats. Is this because the more expensive one will overtake the cheaper one eventually as he will learn faster?

Also, I have an average looking player who is also worth over 1m (See pic below). Why is he so expensive with average stats?

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These two players are my most expensive, but I have better players in terms of stats for their age. Should I sell them?

Thanks!

The more expensive player is a faster trainer.

Also it looks like the higher quality player as come from a higher level academy both as 2* and released.

Easy way to check put all three head to head on a full training session starting with conditions and moral equal (full 99%) then check which one has the highest gains in all three exercises.

What would you do if you were in my position? Sell them both? or keep them and train them up? I have better players that I think I should use instead.

I bought the better quality GK and was going to use the lower quality one as my sub. Do subs learn just as fast by giving them training instead of match practice?