Actually, if you are using the default supply rules, the supply doesnt flow from 6A, but from OKH direct. It probably still follows that road, however. When you click on the supply layer, it shows the layer based on THAT HQ selected...and all clicking on the map will do is show a route from that HQ. (and green, yellow, red, blue hexes for amount of supply that gets through...corresponding to 100%, 75%, 50%, and 25%...not necessarily in the same order)
I am beginning to suspect, now that I am playing with the expert supply rules, is that supply gets 250AP's to move from OKH/STAVKA to get to the unit. It flows first to the Front/Army HQ. Then it flows to the Army/Corp HQ. Finally it flows to the unit. If the Front/Army HQ is too far removed from the Army/Corp HQ, then you can be adding a lot of kilometres (AP expenditures) to the supply route. My (german) units seem to have to be within a couple of hexes of the Corp HQ, and the Army HQ is best near a road/rail line.
“My logisticians are a humorless lot … they know if my campaign fails, they are the first ones I will slay.” – Alexander the Great