Check to see if you have smart-recording or auto-pause turned on. TrainingPeaks is not compatible with Smart Recording or auto-pause, as TrainingPeaks uses every second of raw data when calculating. For the most accurate data, we recommend using every-second recording and turning off auto-pause.
By default, most Garmin devices have Non-Zero Averaging turned on. This setting throws out all the zeroes when figuring Average and Normalized power. TrainingPeaks does not throw zeroes out because time spent coasting is resting, and rest allows you to hold a higher average power when you are pedaling.