A few results to compare the estimated journey times. In all cases the source and destination were the same and all three engines planned the same route. Pure Maps profile Offline, maps OSM Scout (default), units British.

Route 1, 12 miles (home to a random nearby location)
Pure Maps: 15 min
Bing: 25 min
Google: 22 min
Reality: around 25 minutes IIRC, heavy but flowing traffic, no delays

Route 2, 34 miles (home to work)
Pure Maps: 37 min
Bing: 48 min
Google: 44 min
Reality: about 45 minutes, light traffic

Route 3, 98 miles (home to our most commonly used airport)
Pure Maps: 1 h 31 min
Bing: 1 h 46 min
Google: 1 h 45 min
Reality: usually between 1 h 30 min and 2 hrs, mostly on motorways, exceeding speed limits (shhh)

OSM Server has its own units setting. This was set to Metric. Changing that to Imperial and running the above tests again showed no difference, so there goes my theory. It looks like taixzo hit the mark.