Considering it most "traditional" to have 12 issues in a "volume", it would have not been so unlikely to have an issue labeled both "V1:13" and "V2:1", but all logic for the actual pairing of "V1:13" and "V2:2" escapes me.
Any idea when the first issue hit the stands? If it was February, for example, they might have reset the volume with the new year.
Again, I'm pulling that out of, well, let's say my "hat," but it seems like something I might be inclined to do in their situation. What I mean is that it might make sense for some publishing reason (taxes?) to know the publication calendar year by the issue number, regardless of whether the book was launched in summer or winter, and whether it was quarterly, bimonthly, or monthly.
That logic, though, might be exclusive of my other hypothesis, though there's also the possibility that they gave up their plans of "squatting profits" with issue 12, thus making that the second volume...maybe.