XFSetCell order of upload

Does anyone know the order that XFSetCells load in a spreadsheet?  I have duplicate metadata key infomation, and I'm trying to handle it by doing some accumulating with some more generic formulas.  I assumed it loaded top down, so the lower cell would be the last value loaded to overwrite preceding ones.  Doesn't seem to be playing out in every case that it's submitting top to bottom.