Strange that the latest Edge ( 530 ) performs inferior to other older Edge models... The other day we rode in TT style over the same route, and the included picture should show the obvious problem.
This is really disappointing, along with the poor tracking ability as mentioned here : https://forums.garmin.com/sports-fitness/cycling/f/edge-530/166694/gps-accuracy
(as we met for the ride at one point, but had different distance to the meeting point will explain the different km's shown, but same segment) all 3 rides purely GPS based speed.