If so, why does scripture differentiate between wives and concubines? Why use two different words? Why have different rules for concubines?
It doesn't, in the Hebrew it doesn't say wife, only 'another'. That could imply another maidservant or another woman in general. This subject is difficult enough I couldn't even start to comprehend things without looking at the original text. Too much is papered over in translation.
There are a lot of things unsaid about marriage. To start with, Ancient Hebrew didn't even have a word for 'marriage' or for 'marry'. The word 'Marriage' doesn't even occur in the OT Hebrew and 'marry' translates from various different words which aren't even all verbs. It doesn't have a word for 'wife', only 'female', but it did have a word for concubine. Concubines are still females, why would they feel the need to have a differentiating word unless they were different?
And there is much unsaid about marriage. One important example is, what creates/starts it (contract? sex? license? ceremony, what?). There is no broad consensus on that; probably because the text doesn't clearly say. Marriage ceremonies are a big part of that, both now and in the past. Yet they are never specified in the law and only obliquely mentioned a couple times in the narrative.