Difference in the way Apex 1.4.1 and Liberty 1.5 report battery charge

[droid2501]

I was using Liberty 1.5 and I noticed that after I would charge it I would unplug it and the batter would drop from 100% to 90% withing 30-45 minutes even after moderate use. I thought the problem was Liberty so I tried installing Apex 1.4.1 because I'd heard it had superior battery life. When I shut my phone (running Liberty 1.5) down in order to get to ClockworkMod to install Apex, the battery life was reported at 90%. After installing Apex the battery life reported by Apex was 100% and it stayed at 100% for 15 minutes. In other words, Liberty seems to be underestimating the battery charge, either that or Apex is overestimating it. My question is; is there a difference in how Liberty and Apex report the battery life? There must be, right? How can one ROM report 90% and then another report the same battery holding less of a charge at 100%?