A bit depth value is not exactly a quality indicator. It just shows the available range of the S/N ratio. For example, if the integer bit depth is 16, the available range is 96 dB which is enough for any practical use.
If the maximum possible volume level of a given playback system is e.g. 110 dB SPL, the 16-bit range starts from 14 dB SPL which is quieter than the background noise level of any normal listening room. On this example system using a higher bit depth like 24 dB would not improve the quality of the sound in the range of 20-110 dB SPL at all. In theory it would improve the quality of the least significant bit of the 16-bit data (which would produce the 14-20 dB SPL range in this example), but even in a dead silent anechoic chamber you would be unlikely to be able to say anything about the quality of the faint sound that can just barely be detected to exist. (In addition, the noise that is produced by the analog HW would probably mask this audio signal anyway.)
The MP3 format does not store the integer bit depth data. The bit depth in MP3 is "floating". Depending on the audio data the effective bit depth can momentarily be anything (like 1 bit). For the most part lossy audio compression is about bit depth reduction. When faint audio signals are masked by louder signals they are more or less replaced with noise. This S/N reduction is inaudible (at least that is the intention).
Since MC's internal processing uses 64-bit floating point precision it is logical to decode MP3 data directly to that format without doing an intermediate conversion to an integer bit depth format. In theory this is a more precise way to handle MP3 data.