Doubt about parameter RC3 TRIM

I got a drome crash these days and trying to check reasons from log files. I found only RC3 …throttle is 1017 near MIN 1016,all other RC TRIM volue is 1519 this is middle of the range.why RC3 is different?
I calibrited RADIO following Mission Planner direction,
the last step request me to put all chennal at center place but the throttle at bottom, I think APM got the TRIM volue here.