A custom order used to be a hack to be sure you got a freshly-made sandwich and not one that had been sitting in a warmer for 15 minutes, but they make everything to order now. And they still fuck them up, often.
Hard to know for a fact without knowing where I live, I'm guessing :) FWIW, it's not true at McDonalds in Spain, they definitely have popular stuff sitting behind the counter for longer than the items you customize here.