Thank you Sam for your reply and please forgive me if my post wasn't appropriate.
I know eleuthria by fame and I didn't want to point -directly or indirectly- any accusations at BTCguild (I actually avoided to cite it). Moreover, that behavior may be common to other pools, so I'm interested in why the luck seems so low even in the long run; BTC Guild probably is just showing more stats.
BTC Guild is not even 5% of the network any more. The only pools larger than 5% these days are Chinese (and private farms). It is possible and has happened a few times where a single bad block for BTC Guild now takes over 24 hours to complete. Back when we were 35-40% of the network, that same block would have only taken 3-4 hours.
This absolutely nukes the luck figures for the 1D/3D/1W/2W, and even has a big impact on the 1-month luck, due to how small we are now. Our 1D-1M figures were over 100% until that bad block hit, and 3M was climbing back up as well. But at this size, you will almost never see 98-102%. Luck "averages out" in an infinite time frame, but when you're small, it rarely IS at the average, it will be one extreme or another, and when you average the extremes you'll probably get close to neutral.