supply question

[Flop]

I have another supply question. In the screenshot, the Russian Xth army and a couple of independent Russian corps ought to be unsupplied, as far as I can tell. It seems that the Xth army is either tracing supply through regions my units are occupying, or through the impassable mountain region to their north west. Moreover, they are drawing supplies from a distance of 3 regions, not 2.

In fact, if I've understood the rules correctly, shouldn't they be isolated? They are completely surrounded (except to the south), after all.