I wouldn't say that PSI loss is unusual over 75 days.
Lots of factors - flight crew may have played with the masks a bit more (ie: a line check or initial line indoc or something), some over-zealous engineers giving it a huff during cleaning? Tough to nail it down. Our guys are supposed to ask us to re-pack the masks in the containers if they pull them out (for training, for fun...what ever). Maybe some guys didn't. (Check your pilots' Facebook for pics with them wearing Ray-Bans with O2 mask trying to look cool
).
Perhaps mask may also be set to 'Emergency'? When they press the test-button on the container it may be spewing out O2 in the mask, instead of just showing the flow-eye-ball-thingy working? Not sure if it would do this though - would have to check.
Our practice is to service when <1600 PSI. Min-Dispatch for our varied types (for 2 crew) is 1300 PSI so gives us bit of leeway at the outstations. Always top off if jumpseating crew or relief-pilot on board.
I'd hate to work in a jurisdiction that doesn't let your service the bottle! Bizarre.