A month is a sign, not a count
Bikram Sambat is a solar calendar. Baisakh begins the moment the Sun enters Mesh (sidereal Aries) and ends when it enters Brisha, and the same rule sets every other month. The Earth's orbit is an ellipse, so the Sun does not appear to move at a constant rate: it crosses some signs in a little over 29 days and others in a little over 31. That is the whole explanation for the 29-to-32 range, and it is also why the number changes from year to year — the entry moments drift against the civil day, and a month that ended a few hours before midnight one year ends a few hours after it the next.