Interesting topic. On a related note, it can also vary when in the same gear:
For example, when I first had my tune, it was showing a rich mixture as it comes onto boost (about 11.3) before settling to almost bang on 12. Took it elsewhere with the same tune, and this one showed the rich patch going to 11 before settling on about 11.7. Air intake temps were showing the same, the runs were both done in third, so the consistancy was there, just showing different results.