The difference is that the U4 line is oxygenated fuel. What that means is that the burning fuel releases additional oxygen into the combustion process over and above that available in the air. Jetting that is correct for non-oxygenated fuels would be leaner on a switchover because of this additional O2, and richening the jetting is the only way to correct for it. All that being said, it should be noted that quite a lot of the pump gas available these days is also oxygenated. That fact can reduce the amount of richening otherwise needed.