I know that battery charge/discharge is very dependent on battery temperature. I don't know that battery chemistry, each has it's own quirks and often they are not intuitive.
I know that lead-accid batteries can be optimized for cold and low altitude or higher ambient temperature, but not both. I know that because sometimes our winters are that cold that battery is having a hard time cranking the engine and then it does not charge in first 10 Km, because battery is too cold. I need an external charger that charges the battery and warms up the engine when it is close to -30C, which does not happen often but there is one to three week cold period. This is where it gets weird: the startting battery does not give easily cranking current or take charge when it is cold, on the other hand it does not seem to loose charge that fast either. The external charger has temperature control and is located close to battery, it charges with somewhat higer voltage on winter than on summer.
Pretty sure that OP observation is least partly caused by battery temperature induced.
Pekka