Visual balance. The empty space would upset the eye more than the cut off numeral. Same reason you see “IIII” for 4 instead of “IV”, it’s to balance the “XIII” on the other side of the dial.
There are other ways of handling it, but it’s one solution.
Thats lovely. Only thing that spoils it for me is the cut off 6. Why not just remove it entirely? This ruins so many watches for me. Good job really