NavList:

It's explained in the Explanation section of the Nautical Almanac. The increments in the 'increments and corrections' table are based on adopted hourly rates of increase of GHA. For each object, v is the excess of the actual hourly motion over the adopted value. For the Moon, the adopted hourly rate is 14° 19.0' (not 15°), so v is always positive. The Sun and planets are lumped together with a single hourly rate of 15°, so Venus (uniquely) sometimes has a negative v.

In other words, it's a convention to make the tables compact, and the calculations as simple as possible.