Another factor is the AH rating is affected by the current draw. The higher the current the less likely the battery will deliver it's rating. I've never seen an AH rating with the current draw it's rated at, so just from conjecture one might assume you are more likely to get the stated capacity on the higher voltage battery, which can deliver the same power with a lower current draw. It would be a simple matter to check the specs on the cells used in the battery assuming you know what they are. One of my eBikes is 36V because it was the bike I wanted at the right price. Which is way more important than the difference in voltage.