1. PPLNS - you will recieve your payments within some time after you disconnect from pool
2. check blocks effort/luck in that time
3. network hashrate growth can reduce your payments =)
...
etc... all that with keeping in mind that pools you've checked are not cheaters.
1. Checked. I know about it. It's not a reason for 2x difference.
2. Largest pool with 80% blocks. It's not a reason either...
3. +-5% And rather it folldown than grew.
So..... Trust to nobody.

))))
heh)) u can try small pools and find your's best... Our for example