It's not really ambiguous. 127.X does not equal 127.0.0.X. 127.X means, the first octet is described by the first number, and the last three octets is described by the second number, and X is not limited to 0-255. Thus 127.256 is equivalent to 127.0.1.0, something that would not be possible to express with your alternative interpretations.

It doesn't just mean "fill in zeros".