BTW, seems like this is a common "issue", TE is apparently calculated by the device. AFAIK, the 210 does not calculate TE so the displayed value is a generic default. See this thread:
seems like Garmin Connect should be able to calculate it
You need R-R interval heart data to calculate TE.
The 210 files don't contain this, only heart-rate recordings at smart intervals. It is not possible to calculate TE from them, only from files from watches equipped to record R-R, which means the 610 & 910xt (though from reading the 910xt forum it may not be possible to enable it on the 910xt yet).