Just a guess
Edit: oh I missed mentioning a second cause of it also.
There's a large miner that fails over often to the pool, his SPS value is actually about the same as the pool total SPS!
So when that happens, there may be a large number of 'Hi' shares when he connects - and as I've explained before about 'Hi' shares, they actually don't matter at all in terms of reward effect on the miner. But alas, they do show up as part of the 'Invalid' count.
So again, when this happens, if the pool Diff% is very low, it will appear as a much higher Invalid %.
WHOA....pump the brakes a sec! Almost missed the scope of this! U are sayin there is a 200+ph miner who FAILS over to the pool randomly? That's a really big fish in a really small pond! Not complaining, just wondering the logic of such a move