I think it's because the person you're responding to didn't seem to be implying that a single X or a single y would count? Instead, they're probably talking ab

I think it's because the person you're responding to didn't seem to be implying that a single X or a single y would count? Instead, they're probably talking about taking three cells with XY, and then using that to create an xx, an xy, and then some leftover parts. If that is the case, your correction might seem arrogant or assuming.