This bug was reported on Ubuntu bugtracker, at https://bugs.launchpad.net/ubuntu/+source/xfce4-battery-plugin/+bug/318913

Version: 0.5.1

The bar that shows the percentage of battery life left does not visually correlate with the percentage shown all of the time. For example, the maximum height of the bar for me is 16 pixels. When I have 49% battery remaining, the bar's height is only 7 pixels. I would think that it should be 8 pixels because 49% is much closer to 50% than it is to 43.75% (7/16). It appears that the algorithm for determining the number of pixels is floor(percentage*16) instead of round(percentage*16).