I went to a different McDs about 1/2 mile down the road. The 2y price was again prominently displayed. Again I was charged 4Y. The good news is that I watched a Chinese person buy a cone, and, by the change, I could tell that she was charged 4 Y. Really strange that the wrong price would be so prominently displayed.
To those making mirth of the small amount, I would say that I negotiate a lot in business, and I always want to understand the culture in deal with. If I, as a foreigner was being charged double, it would be revealing in a negative way. Glad to say the potentially negative connotation was not true.