Why are Sotaroas cheaper than Toxotai?

[QuintusSertorius]

Err, my EDU says that both cost 411 mnai.

Edit: actually, looking more carefully, there are two Sotaroas units with different costs. For Arverni, Aedui and Casse they cost 411. For the rest they cost 392, but have 1 less armour point.

These are regional Sotaroas (I'm playing Epeiros-as-Massila) compared to factional Toxotai.

[The Celtic Viking]

These are regional Sotaroas (I'm playing Epeiros-as-Massila) compared to factional Toxotai.

Yes, I realized that, hence the edit.

In any case, while the total defence value is the same for regional Sotaroas and factional Toxotai, the Toxotai has one higher armour value but one lower in def skill. If def skill cost less than armour, then that can answer for some, and I believe Sotaroas get their 1 armour value from being Celtic, which might even mean that its value is free. That, along with worse modifiers and apparently a slower speed, might help explain the difference.