If you are a Strava subscriber or whatever they call it - pay money - there's an analysis button. If I go there and look at calculated power - I don't have a power meter - I see that the software has created power surges, I suppose one for every time the Garmin updates position. The exact location of these surges vis a vis the ends of the segment will cause different power estimates, especially on a short segment.

Those high wattages are the peaks of those surges. If you really want to track power, you have to have a meter. IDK about the 19 mph. Haven't had that happen.

For long segments, Strava watts estimates track very closely with those provided by online bike calculators, without the hassle of inputting data into said calculator. They should, because they and Strava use the same formulae.