Neville -
I read your article about the spurious impulses, but I don't think it applied to my measurements.
The 2 rides I wrote about in the original post for the half-marathon measurement were my 3rd and 4th rides of a couple variations of the course in March.
The segments that generated what I believe to be missed impulses were also the ones that involved the longest downhill sections of the course and the fewest number of stops. Only one of them involved a single stop, and the others were ridden continuously from the start to just before the mark, then rolled forward through the mark (just like the previous rides).
Because I used 2 magnets, it's easy to tell where the rear magnet is by just looking at the position of the forward magnet.
When I get a new counter (and presuming it will fit my wheel), I will compare those 2 methods during a free weekend and post the results (likely not before May 10th, however - race scoring season has started for me).