The observant amongst you may have noticed this tiny luck addition to It shows what the probability is of reaching this pool difficulty and is a surrogate marker of "luck." We are a long way from the worst luck the pool ever saw which I searched for and found was on Christmas day 2016 at a truly horrendous 789%.
Footnote: This value is never perfectly accurate because it does not take into account diff changes over the block solve duration, underestimating it when diff is rising, and vice versa.