Is there a chance that is what happens when a larger miner has a bank of machines go offline?
Just a guess
if it's offline, it's not returning anything. Would have to be when they come back online with a bunch of "stale" shares.
It's probably due to looking when we find a block.
Every block change, the first shares are stale since they are the ones that arrived late from the previous block hashing.
However, when we get a new block, all the stats are initially zero, so the stales are initially a large % of all shares.
The total stale % goes down as more shares are submitted after the block change and goes up a bit every time there is a new network block - since that's what causes stale shares - but as time progresses from our last found block, as we get a larger and larger Diff%, the amount it goes up is of course relatively a LOT smaller.
You'll see it is currently down to 0.35% (at 5.8% diff)
Hmm - looking at that other pool - 2.8% rejected at 73% diff ... ouch.
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 %.
so it varies up and down a bit, and varies more depending on how variable your hash rate is. 
