@Morphoice because the midi number node output is 0-1, as it's essentially giving you noteNumber/127. So you multiply by 127 to get it back.
I have no idea why the faust node parameter rounds it erroneously between C2 and E2.
PMA takes the value, multiplies it with the multiplier and then adds whatever you plug into add. The base PMA node aims to output 0-1, while the unscaled outputs anything.