I don't know that much about how it's done, since I'm a bit new to this gig, but I switched to minergate and their numbers didn't seem to add up with the amount of hashing my systems are producing. A discrepancy of about 30%. I'm not saying they're doing something crooked, but I switched to some different pools and the numbers match up against my calculated stats again. Maybe it's a difference in the reporting, but I remain suspicious.