@aragon82:
To help explain your special case a bit better: The guild ~Dragonstar~ has been in a battleground with 7 participants, while your battleground only contained 5.
The more competitors you have, the more MMR points are at stake (so you can win more, but you can also lose more). This is simply done since the more competitors you beat at the same time, the more statistically relevant that result is in the big picture.
For live worlds - as I said before - you may have a totally valid point that we need to reconsider the initial MMR allocation (upon releasing the feature), so that the swelling into Platinum and Diamond won't take too long.
For your interest: This whole discussion lead us to have a closer look again at the Underdog bonus and if it worked correctly. And in fact, there was a minor bug. But it didn't affect your specific case at all.
So it was really helpful that this was brought up.