My Surf PI 1.2 sample width vs. ITMD example has me confused - what am I missing?

[Polymer]

I've had my oscilloscope for a week now and have been looking at many curves on the Surf PI 1.2
and trying to make sense out of what I'm seeing.

In this case I'm looking at the sample width (Pin 3 U4), which seems to be around 40us.
A lot of that covers the flat part of the...