Why does Vapor Fuel increase exhaust oxygen?

Better combustion (true combustion enhancement) increases exhaust oxygen in several ways.
First, when you use less fuel to do the same work, there is less fuel to oxygen ratio and thus a higher oxygen content in the exhaust.
Learn more about why and how fuel can be reduced download Double Mileage Guaranteed (Part 1)
Second, when combustion is complete, there is almost no carbon monoxide (CO) and almost no oxides of nitrogen (NOx) in the exhaust; so the oxygen that would normally be ‘tied up’ as CO and NOx is ‘free’ and shows up as ‘increased oxygen’.
So, as I’ve stated, ANY true combustion enhancement will cause increased oxygen in the exhaust.  Vehicle computers CANNOT ‘understand’ that this increased oxygen is OK, because computers are programmed to increase fuel consumption whenever the exhaust oxygen content rises (they ‘assume’ the fuel mixture is too lean).
Thus the EFIE ‘corrects’ this computer inadequacy by making the additional oxygen ‘invisible’, so the vehicle’s computer operates normally and doesn’t inject ‘additional’ and un-needed fuel.