Some audio devices do not accept a native 24-bit signal, but instead require 32-bit - in this case MC takes the 24-bit and pads it with zeros until its 32-bit, and out comes 24 (padded).
Its still bit-exact 24-bit audio, just padded to fit the requirements of the hardware device - which is done automatically if the hardware requires it.
Note that there is still a distinct difference between 24 (padded) and actual 32-bit in WASAPI, as the audio device is told that its receiving 24-bit padded instead of actual 32-bit audio, so it can act accordingly and properly process the 24-bit of actual audio - the padding is just for transport.