In general, because one byte can only handle 256 discrete values, so you would only resolve to 0.015 volts on the 1-5V range. Using two bytes, you can handle 65,536 discrete values. Two byte values (16 bits) can therefore theoretically resolve to 0.000061 volts on the same signal.
[edit]
/sigh. I didn't realize the poster posted multiple topics on this.