Back to my original post...my split of hashing with BTC Guild and Slush were equal for 5 weeks. But the last 8 days, BTC Guild payed out 3/5 of what I made here at Slush for the same hash power on both.
Likely your extra fifth was where luck fell but Guild hasn't had the best either in that period. You can try and chase luck but unfortunately luck just comes when luck comes.