I don't know why they do it - Normy stands over Dave but she also did similar to my OH when he moved in she didn't stand over him as he was never on the floor
but she used to posture in front of him in the front room if we were watching telly. She'd stand really proud with her back to him and legs slightly splayed showing off her muscles but she's never tried to dominate any of us and for all her posturing, if anyone has an upper hand in the dogs relationship I'd say it was Dave but that's only based on the fact that Normy lets him take things from her - food, toys etc.
I'm not sure there is always a dominant dog in multi-dog households, there is no obvious dominant dog with my two they're both quite strong personalities that have worked out how to live together and be polite with each other. Watching them wrestle is wonderful they've both learnt moves from each other and take it in turns to be the winner it's incredibly fair and lovely to watch, although Dave's spinning body slam isn't quite as effective as Normy's