Thanks to The Travellers' Digest #6 and 14 we have XBoat routes for Lishun and Magyar sectors. As noted on the Traveller Sector Information page, the Magyar routes don't distinguish between Solomani and Imperial borders. In general, though, the DGP data is pretty good, so I assume there is some simple explanation.
Interviews with Joe Fugate (possibly in this thread, maybe in TD#21 or MTJ#4) indicate that when DGP was taking ownership of the Traveller Universe they put copies of the entire Atlas region up on the wall and started plotting all of the mains. The XBoat routes I've transcribed so far seem to confirm this attention to detail - the routes do not follow logical trade routes (as some have agonized over) but do connect mains. It is important to remember that the purpose of the XBoats network is to quickly spread information over long distances using fast (Jump-4) ships. It must be assumed that information will flow along the mains via J-1 and J-2 traders, a week or two behind the XBoats - meanwhile, the XBoats have spread the news 4-8 parsecs further.