HP 54710A User's Reference Manual page 196

Measurements
Finally, Rising and Falling Edges are Determined
If there are not enough points on an edge, the rise time measurement, for
example, is either not made or is flagged as questionable.
The oscilloscope scans the waveform data and records the transitions
through the three thresholds. If a waveform data point does not exactly
correspond to the threshold level value, the system interpolates between the
two points about the threshold. Then the location of the transition point is
recorded in an edge array. The system ignores incomplete transitions and
glitches as shown in as shown in figure 13-8.
Figure 13–8
The system ignores incomplete transitions when defining edges
13–11