Time estimate varies depending on map

I had created a route for a club ride. About 375 Km. The time estimate using the HERE Gold map is 5h 49m. When I sent the map/gpx to another user, he reported the time estimate was 6h 19m. I tried switching to the general map and got the same estimate of 6:19. I then switched to the TomTom map and got an estimate of 5:18. That's an hour less than the general map. The distance remained fairly consistent and stayed within 1km for each map.
Why is there such a big difference in time estimates?

@DougRobinson Each map calculates the route in its own way, and with its own errors. As example, one map could take into account a roadblock while another doesn't.
The same for GPS deives that use different maps and calculation algoritms. Hence it is normal that calculating a route with different maps gives different results.
As a golden rule: always calculate the routewith the same ap that your device is using: TomTom or HERE for Garmin.
When riding in a group with different devices, the mara routeplanner allows to compare routes so you can add/move waypoints until it is exactly the same for both device families