Ethanol still has carbon. It’s a carbohydrate rather than a hydrocarbon. So it’s not 5% less carbon. And it’s certainly not 10% less fuel economy going from E5 to E10.
The difference is where that carbon comes from. Ethanol is made from plants that got that carbon from the atmosphere in the last year, so the net amount of carbon in the atmosphere remains in equilibrium. Petrol comes from oil that has locked up that carbon for countless millions of years, and so the net amount of carbon in the atmosphere increases.