This is my guess. And it is only a guess as I am not familiar with the DAW you're using. I use Nuendo and I am basing my answer to you on that software.
If you have a 24 bit file, then that is its final bit depth when the file is stored. But when working in Nuendo, all is processed using "32 bit floating." You need more bits than 24 for increasing level, adding EQ. etc. If your file is maxed out at 24 bits how could you increase the gain of an entire mix without more bits? So, it is the increased number of "processing" bits that allow you to change your audio and extend its audio level past "0," as there is a much higher "0" with the processing bits. The bits are truncated or re-dithered to 24 when you export your audio from the program you're using.
Now, if I have this wrong, a much smarter person will inform you (us) better!
Barry