Difficulty has almost tripled in the last month (as in +200%) from ~155 million to 550 million. Everything else being the same you would be making 72% less. If you started a little earlier and the initial difficulty was ~100 million you will be making 82% less.
This is very important to understand when you are mining. "Difficulty" is not just a word without meaning that people drop.
I have some miners who don't believe the difficulty is real, and they claim I am stealing their money. They claim they should always get the same pay for the same hashrate. I have others who believe I am the one controlling the difficulty and that I'm cranking it up just to make their lives miserable.
The difficulty is very real and it is an automatic mechanism in Bitcoin that pool operators do not control. You need to understand difficulty if you are mining.
Measuring the luck of 4 different pools is useless. Thinking that the luck yesterday will affect the luck today is called gambler's fallacy. You can read about it on Wikipedia:
http://en.wikipedia.org/wiki/Gambler%27s_fallacyMeasuring the income from different pools as you move between them while the difficulty is changing... is worse than useless. The difficulty going up 50%
will have an effect on your mining income - there's nothing any pool can do to eliminate that effect. Going back to the previous pool you will find that earnings have dropped there too.