parshanut torah comment - Zombie Midianites?

In Numbers 31, the Jews wipe out Midyan. I'd think this referred to only those of Midyan who happened to be defending against the Jews' attack, and not the whole nation, except that even women and boys were killed, who presumably don't go to war. So, seemingly, all of Midyan (except the girls mentioned in verse 18) was wiped out. Yet, we see Midyan fighting again in Judges 6. What gives?