Thought I'd share this little bit of field research. Any suggestions as to why this is happening?

I run a T6D mostly for gym work and an Ambit pretty much for everything else outdoor and I was curious to see how they recorded activity/effort in comparison to each other so at a spin class last evening I wore both.

The results were as follows:

Duration PTE kCal Ave HR Min HR Max HR VO2 Max
T6D (postworkout in device readings) 42.57 min 3.9 646 139 101 162 39
T6D after upload to Movescount 42.57 min 3.7 715 140 101 162 39
Ambit (no change in data after upload) 43.07 min 3.8 683 141 101 162 39

I do understand that the T6D 'in device' readings are modified by Movescount on upload to be more accurate (Ambit's remain constant pre and post upload) and it's reassuring to see HR readings remain constant between both devices and pre/post upload with the exception of averages - but even those figures are pretty close. I wonder however how/why the PTE and kCal figures vary so much. Surely as both devices use the same 'personal' data and the device reading are in effect identical, why the discrepancies?

Oh and the slight time variation was my fault as I was distracted, but surely 10 secs (0.38% variance) cannot account for a 4.7% variance in kCal. PTE is a more difficult to comment on and a borderline reading could easily move up or down .1 as there is no intermediate decimal places.

Any thoughts?