You're seeing it exactly correct. Adding a calculation for actual volume will likely be part of the updates Brad is working on.
Think of it this way, though. When you're buying supplies, you don't want to be short. So, purchasing on batch size or more is prudent. You currently have a carbonation tool to use the real volume, storage temperature and carbonation target to get a precise amount.
In the long run, updating all your profiles with your actual measurements is the best way to eliminate confusion and frustration. The closer your equipment profile is to your actual performance, the more accurate the predictions will be.