I saw this in Krabi and it was a bit of a turn off at first as who would find sand appetizing? Of course this provided much comic relief in the end! The only thing I can say is that this misspelling of sandwich is due to the reliance of the shopowner (who probably does not have a good grasp of English) on the how the words sound and writing it as they hear it. Just like how cheese is spelt "chees" in the sign!
Being really curious about how the word "sandwich" came about, I googled and found this:
