I’m betting that to keep novices from overloading, Zoom, Tascam, etc. set the input gain of their recorders much lower than customary, say, 10 dB, and since the signal to noise ratio of modern day converters is so good, they can get away with it. Then convert the output of the ADC to float, at which point they check the output level and raise it if it won’t clip.
Then, Zoom has a few options:
a) output a 24 bit fixed point file…. If they don’t dither at that point to 24 bits then at least theoretically they’re doing a bad thing.
b) output a floating point file that represents the gain raise that they may have performed.
c) Or make no gain change at all, just capture the attenuated signal from the attenuated ADC and send that unaltered in floating point format to the user. The user can raise the level of the floating point file after downloading it from the Zoom unit — without penalty. At some point the user has to have a little smarts to know what they are doing, Zoom can’t protect them cradle-to-grave from their own ignorance.