The Optimate is a low current rate charger, 0.8A I believe. and a bit undersized to charge a car battery. Probably good enough to maintain a fully charged car battery which is what I take it you were doing.
A battery can boil/gas off below its per-cell max charge voltage if it is defective. You do need some ooomph from the charger and I would be surprised that the Optimate 3 could do that in a short time period. BTW, most of a battery's weight is in the lead.
Its unfortunate I can't comment on other battery chargers other than the ones I have tested, most of which was done at the defense labs where I used to work (now retired so I can ride more...wink). Some of the big & well known names did poorly.
I ran those graphs above using a Madgetech data acquisition module that is the size of a small matchbox. If anyone has an on-board voltage issue, it can be used to monitor voltages (or anything else that can produce a voltage).
I wouldn't have an issue doing some testing for the ON, but I have so much on the go usually, I'd hate to make a promise I cannot keep. Plus I have the engine/transmission out of my 993 for some minor items.