Player Value

So something I have realised is that my players' value keeps decreasing compared to other players? is it just me ? or is it something im doing wrong?

For example, Torres starts off at 58 million euros, after 1 season, he scored about 25 goals and his value still went down to 20 million.
John terry decreased from 16mill to about 10.
and the same for a few of my other players, any ideas if im doing anything wrong or is this normal?

is it something to do with training maybe or their contracts?

well decreasing value is always gonna happen more or less. age, contract, reputation these things have an affect on it. contracts from what i remember if they are committed long term their value should be high as well. doesnt mean it will go up though. also a lot of starting values for players are from when they have been bought. so u buy a player for lets say 25m his worth will most probably drop. its not like he will always be worth 25m especially if you paid way over his worth. age obviously matters, no one wants an old player.