First off I don't know the answer, but I had a little play with one of ours today, a -900 set up to FAA performance FWIW.
Actual ISA dev. was -2C, and it would accept up to -20 C. This didn't change the optimum altitude, but changed the max alt by 300 ft.
OS is probably correct as he usually is, but I would hazard a guess that it's a certification issue to ensure that you can't "fool" it into trying to climb to a too high an altitude by falsifying the temp. I would also hazard a guess that a CAA/JAA/EASA performance certified aeroplane, would be more conservative than one certified to FAA standards.
Interested to hear from others.