Variance old bean. The time taken to solve each share everyone is sent is random, but has an average related to the difficulty. On smaller pools the variance will be huge because there's so few workers and they could easily have a bad streak. Buy just as likely they can have a good streak. On larger pools you have less variance because you have much more workers so bad streaks/good streaks are averaged out. But it doesn't matter which pool you use because over a long time span they average out to your expected earnings (not taking fees into account), just depends whether you like frequent small payments, or infrequent large payments.