It's because in ancient times when PvP still wasnt a meme, there existed two legendary modes, The Fold and The Feast. In SE endless wisdom they made new achievements solely for the Feast starting by 0. Only later they were so annoyed by the Fold that they removed it. But by doing so those achievements would have become unobtainable, so in their generosity they made them count from now on for The Feast as well. And of course they won't track any previous Feast matches you already had. What this tells me is, you played at least 4 matches of The Feast, before The Fold was removed. I am in the same boat, I have over 5000 wins from other achievements, but it counts only 4000 something.