I guess that's exactly what I don't understand. If I add a 32-bit integer and a 64-bit integer, what other possible result could I be expecting besides a 64 bit integer?
A 128-bit integer (if you are adding a 32-bit integer and a 64-bit integer, the smallest power-of-2-bits representation guaranteed not to have an overflow is 128-bits, so its the safest result. Though, I'd agree, not the most likely thing most programmers would intend.)