its because of air temps and header design.. . "the front cyl is cooler. a colder air charge is denser, needs more fuel"
you can tune the rear cyl map (its got the 02), copy it to the front then add 2 points over the whole map. this is with ECMSpy, not sure if the value of '2' would be the same in the directlink world. but the front map needs to be richer.
if you can get another bung welded to the front pipe, you could tune both individually. this is optimal.