can anyone come up with the reason for making the distance 15 nm vice 30 nm?
Yes, here's one reason - old system, westbound tracks: Aircraft on track B has a problem and crew decide to offset 30 left, meanwhile another aircraft on track C has a problem and crew decide to offset 30 right....and..........
( Sounds unlikely but
AFAIK the change came out in the wash up of events on the NATS during 9/11)