The specifications for time interval measurements lists a negative value in the results range. Why?

The time interval measurement on the 53131A/53132A and 53220A/53230A universal counters are specified as follows:

The time interval measurement is specified over the full signal ranges of Channels 1 and 2. The results range is -1 nsec to 10E5 sec.

The counter will display a negative result (down to -1 nsec) if the end on the second channel slightly proceeds the edge on the first channel. If the disparity gets too large, then the counter will change to a positive value.