When I look at the stats from a bike ride directly on my Garmin GPS 305 and in the Garmin Training Center software, they differ slightly from the stats that appear when I upload the data to http://trail.motionbased.com. I was wondering if anyone can explain the differences.

For example, on my ride yesterday the GPS itself and the Training software say the following:
Distance: 52.07 miles
Total time (this is moving time): 2:58:51
Total Ascent: 2235 ft
Total Descent: 2237 ft
Max speed: 37.7 mph

For the same ride, Motionbased gives me the following stats. Note that it gets the stats from an upload from the GPS - seems like they should be the same.

Distance: 52.12 miles
Moving time: 2:58:40
Elevation Gain 3260/-3261
Max speed: 44.3

Can anyone explain the differences?