Just to add to Dave's comment. My understanding is that the monitoring of the catalyst efficiency is not just an "instant" test. By that I mean that the catalyst is checked over drive cycles and includes monitoring of warm up time, time spent up to temperature, ratio of pre / post lambda cycles, OSC, reaction time to ECU induced test routines etc. I am sure there are many things we are not aware of.

Software "easing" of tolerances is common for many issues and should be considered along with assessment of all the live data and scope readings.
A bad catalyst is relatively simple to spot but sometimes it can be difficult for the customer to understand that the catalyst is inefficient when it has just passed an MOT

Hope everyone has a good Christmas and a happy new year.

Steve