People say there are two genesis creation myths "mashed" together. I think this is true, but not really giving enough credit to the people doing the mashing who I think - as the rest of the literature pans out - not generally in the habit of writing directly contradictory things (usually you have to dig deeper for mistakes, but blatant surface level ones are not so common). They didn't have science, but they knew how stories worked. I think it's possible the original editor, when joining the myths together, intended the "man" created in myth 1 (the sixth day) to be understood as humanity in general. And myth two following on immediately is the specific creation of Adam on the eighth day. That would at least explain why Cain's wife is referred to so casually (as well as the implied peoples Cain is afraid of)