BTC Guild was the first pool to deploy variable difficulty, and if you do not adequately set your minimum difficulty, the pool will adjust it for you. That is why the setting is labelled "MINIMUM difficulty". BTC Guild uses a 20-40 Shares Per Minute range (it doubles your difficulty any time you hit 40 SPM for a 5 minute duration).
I "adequately" set minimum difficulty to reduce discarded shares. Maybe server needs more bandwidth? You are after all pushing 300'000 GH atm.
Discarded shares are meaningless for stratum. The statistic means absolutely nothing to you unless you're on getwork. Just ignore it.
Higher difficulty means higher stales/discarded, how is that meaningless?