Tudor 79170 accurate except....

[iclick]

My Tudor 79170 Big Block chrono was serviced last year immediately after purchase and since then has been generally very accurate and consistent, around 1-2 sec/day fast, as long as I'm wearing it. When off my wrist it is inconsistent and always runs slow regardless of the resting position, anywhere from -4 to -27 in varying positions on my Timegrapher with amplitude often showing often in the low to mid 200s, but stays around 250° when warm. Room temperature has been 68-75° lately.

I assume this is a temperature issue, so what is the cause? My other BB chrono with an identical movement is much more consistent across these temperature ranges.