HP 54710A User's Reference Manual page 218

Measurements
Time-interval measurements
For time-interval measurements between edges of opposite polarity, the
errors caused by dc level errors add rather than cancel as shown in
figure 13-14. The time errors in locating the leading edge and trailing edge of
the pulse reinforce rather than cancel.
Figure 13–14
Edges of Opposite Polarity Reinforce dc Errors
13–33