Just to emphasize Garmin's can be inaccurate in regards to top speed. I've looked at my Garmin at the end of sprints and seen a top speed that does not appear on the ride report when I upload.

Related to this my Garmin has occasionally shown 27% grade climbs! Please note where I live.

Why? Above explains it pretty well. W/o getting technical I always figured the top speed at the last second of a sprint wasn't held long enough to be "official."

FWIW I subscribe to the "doesn't matter"school.