They don't always bother. I have been seated in the first row of M with a headrest cover that said "Economy Flex", but they still didn't give us any food. It seems they keep an internal list of where Y stops and M starts, which doesn't have to agree with the headrest covers (on recent flights I've noticed a little "Economy Flex stops here" sign put on the last seat in "real" Y, that would be even easier to move).

Actually, one guy on the flight above had 9A, the last row of "real" Y, but he accidentally took 10A when boarding. When the woman who had 10A appeared, she offered to switch seats with him and he agreed. He didn't look too happy later on, when he realized he had given away his Y meal and drinks as well. The woman seemed to enjoy it though.