The answer to both this and your preceding question is "because it does more damage". That's why we have spreadsheets - to figure out which does more damage without having to rely on our gut instinct as to which "should" be better.
Hm okay, dont thought that ArP incrases white dps so much that it will beet FotfF. And why is the Spreadsheet telling Agi > AP? Can someone exlain this aswell pls ?