An older lithium battery will show greater dips in voltage under load than a newer one, and will take longer to "relax" to the true voltage state.

I think it is a very safe assumption that there is a discharge curve somewhere mapping voltage to battery percentage, and that any (even temporary) dips in the voltage would show up as a deceptively low percentage remaining - while not reflecting the true remaining capacity of the battery.

How many people with such unusual battery behavior have a young battery?
How many of these anecdotal reports of "Version A was fine, now Version B is acting up" are backed by reloading Version A and seeing if there is true causation or simply correlation with time (age of battery) being the underlying culprit?
How many of these anecdotal reports of unusual battery behavior are comparing stock like-to-like? I've only had my IT for a couple of weeks, but I did not realize swap was part of the standard config.