I believe there is some algorithm that recognizes that if repeated measurements were taken within the range of the GPS's accuracy at the time, it presumes you weren't moving.  I.E. if your accuracy rate at the time is +/- 10', and you had a sequence of measurements that were all within 10', it's going to presume you weren't moving.

It's important to realize that the only truly accurate measurement of distance is by manual means. If you really need to know within an inch how far the distance is between point X and point Y, then you'll want to walk it with a measuring wheel. Otherwise, when using a standard, consumer GPS, being off by 10 feet when you're measuring miles is of no real consequence.

When I publish an article about a trail, I note that all distances are approximate, and they were measured with a GPS. The next person who does that trail, and is using my article as a guide, will find that their distances may be slightly different, but in reality, when they get to a point as measured with their GPS, they will likely be within feet of the spot I described in my article.