Using everytrail for iphone: the app shows for this particular trip 33km in 1 hour and 19 minutes with a resulting average speed of 25km/h.

The same trip displayed on is 1 hour and 46 minutes long, however, which makes for an average speed of 18.6 km/h.

I have paused the tracking after having arrived at destination, then resumed for the return trip. I think that does not understand the timecodes in the gpx file.

This is easily proven by loading the gpx into another application. I have done so (using rubiTrack), and in fact it shows the same duration as my iPhone: 1 hour and 19 minutes.