I can't help with the "why" of this as I don't use Audition, but whether the 16 bit vs. 24 bit matters is debatable. Assuming a good resampling was employed in the conversion from 24 to 16, "objectivists" will say that there's not going to be an audible difference.
At the end of the day, the only way to know for sure if it makes a difference for you is to do a proper ABX comparison.