Pool luck is not really a measure of luck at all. Pool luck is a statistical calculation, like a probability; so for any given amount of hashing speed at a given difficulty level you should expect a certain return. This is like tossing a coin, if you toss it 50 times you should expect 25 heads and 25 tails, although you may not get exactly these proportions.
That's it plain and simple pool luck is a measure of probability and as we know from tossing coins (or probability studies at school) the more times you toss the coin the more chance you have of getting an equal number of heads and tails. Probability tells us that if we toss a coin twice we should expect 1 head and 1 tail, if this works out then our luck is 100%. Lets toss a coin 100 times, imagine we get 50 heads and 50 tails, again our luck is 100%, because we are getting the result that probability says we should get.
If the pool probability calculation says we should expect say 5 blocks in a day, then if we get 5 blocks out luck is 100%, if we solve 10 blocks a day then our luck is 200% and if we solve only 1 block a day then our luck is just 20%, but as with tossing a coin the more times we try the closer the overall result comes to 100%.
It is not surprising that Slush runs at about 100% luck over a long period of time as that is what you should expect from every pool, in fact any pool running significantly below 100% in the long term is probably cheating you as probability says that all pool should run at about 100% in the same way that you should get an even number of heads and tails.
for the noobs out there I will say that there will always be a variance in probability, in just the same way that you can maybe sometimes throw 5 heads in a row; this is normal and unpredictable, you could try chasing the variance by switching between pools in an attempt to only mine in a pool that is experiencing good luck but the chances are that you will be using pure guesswork to base your decisions on, and even though in the short term you may make some gains eventually the rules of probability will hit you and you will have a run of bad luck which will even your luck out to about 100%. The only real option is to choose your pool (or pools if that is what you want) and stick with it, you can console yourself with the thought that whenever you see a run of bad luck it is just getting you closer to your next run of good luck.
I hope this will make sense to everyone and stop people seeing conspiracies whenever the pool luck drops.
CONGRATULATIONS SIR,
That is the best explanation of how this thing works ( or should work ) that I've ever seen on the internet.
Plain and simple and in terms that anyone can follow.