With regard to negative #'s. Note - Zero is not the best figure which can be earned. e.g. a -2 (negative 2) is three points faster than a 1. i.e. 0,-1,-2.

Negative numbers are very positive performances, generally speaking. The domain of top Allowance animals and Stakes horses.

I suppose over the years as horses got faster, especially in the steroid era, the thorograph scale had to accommodate the phenomena. They could have modified the scale but that has complications comparing #'s pre and post, scale changes.