So how come I constantly encounter 47k rated parties while my best five hero rating doesn't exceed 43k?
It's more important to select opponents that don't cheese. Like fighting some teams in standard arena are easier than a team with many nukes, like ethera/cara/ariel will ultimate way too fast... So pick teams that can survive.
Despite whatever hengli said, you virtually can't compete against lineups with 3-4k higher rating, that include heroes like Bloodspear, Dokras, Jasmine and Gizmo with their energy bar half way filled. They will give you a guaranteed whooping, regardless how great the content of your attacking lineup.