As already discussed.
Possibly because the dithering will be avoided.
If signal processing is any more complicated than simple addition or subtraction, there is no simple generic explanation. Signal processing may involve conversion between time and frequency domains and sample rate changes, which are non-trivial in execution and explanation.
That is, conversion from input ADCs and to output DACs.
Upsampling refers to sample rate changes, which may be done in a plugin (most UAD plugins internally upsample to 192k for their calculations, and downsample at their output).
Remembering that 32 bit float and 24 bit integer have the same value bit depth, then the more the 32 bit values are above 0dB, the more least significant bits are lost when converting to 24 bit.