Duane,
The "slightly short" was based on using a pneumatic tire to remeasure certain sections of one of our marathons, and I think a few other courses that were on pretty rough pavement. I found that what I had marked out as a mile, when remeasured with a pneumatic tire, was a few feet short of a mile. I can't remember how short but I think it was dangerously close to wiping out the SCPF.
There could be lots of reasons for this, and I didn't make a careful study of the phenomenon. I regret that now but there it is.
It's encouraging that you get such good agreement with your co-measurer using pneumatics. I have more questions about this but it's probably another thread!