afni.nimh.nih.gov

The formula for converting between t and Pearson r is given on one of Gang's pages:
[afni.nimh.nih.gov]

You can see that t<->r conversions depend on the DOF involved-- so, that can have a big effect (which the r<->Z conversion doesn't have), esp. if you have a large value. Perhaps that is a root of difference?

--pt