An A320 flight crew; distracted by VNAV systems training; fail to make a crossing restriction on descent into ORD.

Narrative

While in the descent for ORD the Captain was giving me a briefing on the VNAV descent function of the Airbus as this was my first leg back in the Airbus after many years flying Boeing. We were discussing the 'doughnut'; managed vs selected speeds and various other items. During this time the 'doughnut' indicated at our speed we would be level at WATSN intersection at 12000ft as planned and assigned. During this time we were not backing the Airbus up with a 3-1 calculation due in large part to our ongoing discussion. When about 3-4 miles from WATSN the circle around the waypoint went to amber and I inquired as to why that would happen as we were still on the 'doughnut' and then we both looked at the doughnut and saw we were now high. The Captain extended the speed brakes minimizing the amount we were off from our altitude but we still ended up over the fix about 700ft high. I have heard people say you cant trust the VNAV function at all in an Airbus and I guess years on Boeing equipment led me to be complacent on my 3-1 calculations as I have never had this happen on a Boeing. I will be much more vigilant now that I am back on the Airbus.