I believe that when you buy E10 (or E15) Gasoline you really do not know what you are getting. Specifically, ethanol absorbs water (we used to use “dry gas” to absorb water and get it out of our gas tanks).
While E10 is transported or stored it is subject to “dilution” with water.
Therefore, when you buy a gallon of E10 gasoline you are buying an unknown mix of gasoline, ethanol, and water. As far as I know there are regulations (State and/or Local) to ensure you get a gallon. But who knows what proportion of the gallon is water. I hope in most cases the water is only a small part, but who knows. E15 will be able to absorb even more water.
I do know that my mileage (MPG) went down significantly when E10 went on the market. This is probably mostly because Ethanol has less energy per volume.
However it may also be due to the volume of water in the gas.