If it is intention then you could explain it as follows: the mercenaries trained and lived every second of their lives as soldiers as the few remains of Iphicrates second unit generation experimentation tool. The lazy fools you recruited as Iphicratic wannabee hoplits from your degenerated townsfolk will never match with the mercs. This is only true for the relation of Iphy mercs and Iphy regulars because it's very probable that Iphy hoplits never existed except on paper.

It's a little bit construed however.