As the screenshot below shows: the data defined position of the text field is working, but that of the table doesn't. Can anyone confirm this behavior?
I replicated this in QGIS 2.14.4 Essen. It seems to act the way you describe until you Generate an Atlas. Up until that moment the expression always returned false, probably because it didn't recognize the columns in the layer. Once i generated an atlas the attribute table "jumped" from 10mm to 200mm. When i then checked the position options it recocgnized the columns and the values correctly.