Pool Estimates are made by looking at past results. That will differ, because block finding is luck. So if you found 20 blocks the past 24h, that desn not necessarily mean, you will find 20 blocks in the next 24h - still you made an assumption that it will be so if the hashrate/diff stays the same (which is never the case). At least, that's how I understand it.
Now, AM seems to compare the estimates to actual payouts. So what they actually compare is how "correct" the pool estimate was. Which again - involves luck and time because they have to wait for maturing at least. The only real information you can get out of this is how CONISTENT that percentage is. So if over a period of - say - >1 week, a pool gets measured at 70% you can assume, that it will be wrong always. If a pool gets measured 70% for just a day it may mean nothing at all.
and beside that, in the past years, the estimates were ALWAYS too high. Always. Every Pool, NH as well. Because those estimates are highly volatile and can never reach a 100% accuracy, which is completely fine, as long as they keep comparable and not being manipulated
If you are on pool A mining but pool B has the luck to find a block - it would've been better to be on pool B. But you wasn't. That's the game.
If someone is interested, I do permanent measurements on earnings and zergpool for example has a 10% rate everyday at 18:00-19:00h GMT, those payouts arrive later on, not sure why this is, but all I care is 24h profit and yes, the estimates don't match. But they never do and never did, at no pool, I've measured lots of them. NH (not a pool but still) has the most accurate so far, which is easy, because they don't need to make any assumptions at all, they have realtime data
The luck factor is important especially if pool hash rate is low, but as you mine longer the luck factor should decreased.
One way to figure things is to mine simultaneously different pools (sharing your hashrate equally between them indeed).
For instance I'm mining LUX simultaneously with zergpool and bsod : so far bsod is giving 30% more earning than zergpool but considering BSOD finding blocks more often I have to wait to see if the difference in earning persists.