"Where" do used batteries lose charge

This is to say, does the battery degredation happen throughout the charge spectrum, or do used batteries drop faster at top/bottom of charge?

Longer version: I have a Valence U27-12XP which I use as a single battery in my Travel trailer (vacation use). Label spec is 138AH, so using 90% as a guideline, I have estimated capacity at 124AH. With this estimation and my AiLi monitor, the voltages have tracked pretty closely with the remaining AH% in the shunt.

I do not know how many cycles or anything battery had before me. I am using a shunt to measure current flow and voltage. I guess my question is... on a used battery that is depleted an assumed 10%... where in the charge is that lost? as in: will it fall from 13.4 to 13.3 nearly immediately in that power is lost at the "top"... or would it, say, be completely depleted by the time it reaches 12V (instead of 10)? or would it more simply just deliver lower current over the entire spectrum as it goes down.

My intention is to better understand the expected behavior of the battery.

thanks,

geometry of battery is a simple matter of surface available to get energy back.
add to this some chemistry and you have the two factor that makes a battery wear.
surface decreasing in efficiency and chemistry not working as good as when it is new.