hum-cancelling is the reason why Brawer stated he flipped and rewired the middle humbucker.
a humbucker, what he actually created, is essentially 2 single coil pickups. except one is reverse wound from the other as well as opposite magnetic polarity.
as stated, jerry used the north coil(closest to the neck) when in single coil. so by flipping the middle humbucker and rewiring it for use of the south coil(as the one closest to the neck) when in the inbetween positions of 2 and 4 on the 5 way selector, he had a humcancelling single coils or in another word a humbucker.
however, rosebud as it lies now does NOT have the middle humbucker flipped so it is not wired like that today. but Gary did state he wired it like that at one time.
"The process of selecting the tone on the guitar is an aesthetic process like any other, so you try a lot of different things. Most of the things I’ve tried I’ve tried in live performances" ~ Jerry Garcia