Hi Pete,
I'm not 100% certain but I think the concept of differential and hysteresis in
the case of these controllers may be the same.
I haven't read the instructions completely, so I'm not sure if in
this case it has a hysteresis effect built in as some sort of fuzzy logic to
monitor and adjust through the history of what occurs over a few
switch on/switch off cycles.
There are two definitions of hysteresis, one is feedback adjustment through the history of what has occurred previously.
The second is simply the loop shape that repeats over time in a graph,
the factors of the shape of that graph being switch on temp/ switch off temp, and the efficiency and thermal momentum of your cooling that
widen or narrow the parts of that loop.
See
hysteresis on Wiki.
Don't forget, there is also possibly a Delay setting on yours:
In the case of a coolroom the compressor (or your peltier in this case) won't actually start cooling for 30 secs (default)
I'm assuming that this is a regulation for power authorities in the event of a power failure so the whole city's fridges don't try to run when the grid
comes back on!
I changed this delay to zero so it cools as soon as I turn it on.
Lower ambient tonight so I set the temp for 10.C, ambient 20C, dewpoint 5c