6v batteries in series mystery

[brettw]

I have 2 6v batteries wired in series for my 12v boat.
Very good and new batteries, Trojan T105, and a very good smart charger, GPC45 MAX.
Problem is when charging, one battery bubbles like crazy and the other doesn't.
The one that doesn't bubble is reading in the " fair " area on my tester. The one that bubbles every time is in the upper "good" area.

Setup is wired correctly.
Terminals are clean.
Plenty of water in both.

Hot battery is at 7.36v
Other is at 6.34v
Both on the charger

Thanks for your ideas.

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