I see that Slushs keeps in 9.5 Ghps from a lot of time. Others pools (as bitminter where i´m moving due razonable fees on getwork), are growing fast. In fact, the named pool grows from 7.5Ghps to 13Ghps in 15 days or less.
This means that some miners are in disagree with slushs services, due some strange calculations, or punisher reward system, and don´t keep here, or move his fast miners to more fairly reward system.
It's mi opinion.
Your observations about bitminter are correct; your conclusions are not.
Bitminter went from 7thps to 13tphs over the past 2 weeks because of 1 thing: ASICMiner has moved miners from BTCGuild (can you guess the reason?) to bitminter. ASICminer is running under a single user and has 6thps on bitminter right now; almost half the total pool hashrate.
That 6thps from ASICMiner is what brought bitminter up from 7 to 13 now; it has nothing to do with people migrating from slush.