I'm not aware of specific differences but there are differences
between manufacturers and between battery types, i.e., starting
batteries vs. deep cycle. I'm guessing a little but my sense of it is
that a smaller battery would be more likely to achieve a 100% charge
and float up to the kind of voltage you're seeing. It might be
interesting to connect a good size marine battery like a 4D in
parallel and see what hapens to the voltage. My guess is that it
would drop as the larger battery should be more capable of absorbing
charge.

I was thinking along the same lines too and bought a 1000 CCA battery. The
motor specs call for a minimum of 360 CCA. So far it has done the opposite of
what I thought would happen.
Someone in the electricial engineering group suggested that I may have been
wrong buying a larger battery because its not discharging from the low starter
demand (the motor runs perfect and usually starts instantly). and the bigger
battery has more resistance.