I once made an excel spreadsheet with all possible training results.
According to my findings, there isn't a difference between anyside and normal players. You just have 3 type of training speeds for players.
Good traning players.
Average training players
Bad training players
(when transfer market wasn't so ridiculous as today, i only bought 10 season - 500CR players, checked if they had the good training speed, and trained them so they were decent from 4 seasons, and elite from 7-8 seasons)
I think in your case this is just a coincidence. because I have some anyside players in my current squad that train same speed as others.
Attacking players (AMID) train the same as normal midfielders (ADEF same as DEF)
deffensive midfielders train the same as defenders. (only tested this once long time ago, so not 100% sure)
In attachements you can find my excel. I even made a simulation how players will grow if I followed a certain training scheduale.