Firstly, some of those AMx/ST players are actually AMx players (internally) with a 'cosmetic' ST position added to them. This sort of practice has become more evident in the game these days with the new auction transfer market. You will see players having 2-3 positions, but when you will check their attributes profile, in many cases, it is clear that the player is just a single position player with 1-2 'cosmetic' positions added to him.

Secondly, there have been a problem with the striker position in this game since a long time. Strikers did not seem to score or perform the way they are expected to do. With time, many users gave the AMx positions more importance, relying on them as if they were the STs of the team, that is those that will score most of the goals. The ST problem should have been fixed by the game company since way back, but only they know why it has not been fixed yet.