My readings bounce around some but generally there is a trend. I rode 12.4 miles yesterday and battery still shows 100%. I'll do the same today and it will lower fairly quickly and again the battery percent will fluctuate with a downward trend. I assumed this was normal but apparently it is not.
I am pretty positive the battery bars are not equal and don't really show the % of charge in your battery. The first bar always lasts a long distance for me with each bar after that going in shorter and shorter distances. I think this is indicated by the width of the bar in the display.