Ethanol-doped gasoline (E0..E100%) has a lower BTU per gallon and a higher octane per gallon, assuming the same base fuel is used (e.g. 91 octane + E10 vs + E15). However, if manufacturers are cheapening the mix (e.g. 91 + E10 to 89 + E15) then they can hold the octane level steady while reducing your miles per gallon by about half the ethanol percentage (so, E15 is about 7.5% lower mpg vs. E0, or 2.5% lower mpg than E10).

You shouldn’t be noticing more than a 7.5% drop between E0 and E15 gas, which would be a pretty hard swing in gas composition. What % do your figures show?