I think the options are:
- withholding attack
- malicious op
- some other attack we don't know about
- ridiculous bad luck streak
Keep in mind Eligius is at 93% luck for 90 days as well. But no one is complaining there.
M
At least a significant portion of the recent bad luck streak was already traced to a group of users that are developing their own ASICs, currently at ~1.8 PH/s. They had previously mined on Eligius back in March and had solved blocks back then. They moved to BTC Guild in early April, after difficulty crossed 4.2b. They were not 1.8 PH/s at the start, they slowly grew over time.
We've since identified that their software was having problems at diff > uint32 maximum value (~4.2b). They patched their software and went to Eligius to test the fix. They solved a few blocks. They had their accounts unfrozen on BTC Guild, and have since found a number of blocks in the last 24 hours for BTC Guild as well.
This being the Bitcoin community, most people are going to assume malice forever, even though these users got back to me immediately after their accounts were frozen when they could've easily proxied their miners and split them up more to make it impossible to trace. They fixed the issue, and have since been reporting block solves as expected.