Aren't you forgetting the pressurization, heating, and cooling systems?

Hilton

Probably not. Bleed air is hot and dry. It goes through air handlers
that make it cool and dry. It still does not pick up moisture in this
process. It then goes into the cabin, dessicates your passenger, then
escapes through existing leaks in the pressure vessel, or through
outflow valves.

Also, at altitude, the system is DEFINITELY in the heat mode, as the
outside air temp is well below 0*F.

The longer you fly, the drier you get. Again, to agree, saying a
particular plane is drier is peculiar. The only thing I can think that
would explain that is IF the amount of air being exchanged is greater
than in comparison to other aircraft. (i.e. greater leak out, so greater
flow in of dry air, which then escapes sooner, increasing the "dry"
effect).

A benefit (if this indeed exists) is that the air is less stale/more
fresh and perhaps less likely to contribute to airborne disease
transmission.. but that is pure speculation on my part.

Dave