Could be that the two fields share such a similar label that the wrong one is being selected when you select the label in the filter builder?
Maybe building the filter via the search center would be simpler. Just double-click the label of field ABC on your contact record screen and enter the criteria in the resulting search center window. With the criteria/results up, you can use the button in the upper-left of the search center to select to Save that as a Filter.
Of course, that's fine for a simple criteria, but if you needed parenthesis or had an otherwise more complicated filter, you'd need to use the full-fledged filter builder and figure out what's going on, field-selection-wise.