Part of the engine warm up software.....
So presumably if the car is not actively driving and has been recently turned on (therefore warming up at the start of the journey and not idling in traffic) the mixture is leaned out to use minimal fuel and speed warming up. Normal settings once active driving detected.
Oops! The test was not specific enough. The regulatory testing schedule accidentally triggers this mode as well!
However the board members were not convinced the regulators would buy this. I wonder how those two engineers were selected?