On Friday, August 12, 2016 at 3:15:21 PM UTC-4, root wrote:

But in practice, they all seem to be the same temperature. How can
that be? Is it just that I'm right but the difference is too small
to sense?

One possible explanation is that the heat exchanger is able to
equilibrate the air temperature at the different speeds you
chose. This would mean the air passages within the heat exchanger
are sufficiently long.

Exactly this. most auto AC units have an evaporator sensor to help prevent the coil from freezing. At a higher fan speed, more refrigerant is permitted through the coil to maintain an even discharge temperature. I had one fail recently (under warranty), it takes about 30 minutes at 95F/70RH to freeze a coil to a block of ice if the refrigerant discharge is not regulated.

Peter Wieck
Melrose Park, PA