What really justifes an mot dangerous defect, really?

Have a think about what I'm saying before anyone jumps in like a bull in a china shop.

Some time back we had a discussion about dangerous defects regarding tyres. ( a privilaged few) Somebody decided because feedback said that tyres at and below 1.6 mm were dangerous that this should be reflected in the fail criteria, i.e. dangerous defect.

What brought this to my attention the other day was that I had two tyres across the steered axle, one was measured below 1.6 mm and the other was very close to 1.6 mm. No problem I thought, I fail one, which is automatically marked as a dangerous defect, but the advisory tyre is not marked as a dangerous defect.

How can a tyre be 1.5 mm and fail for a dangerous defect yet a tyre at 1.6 mm passes the test and is not recorded as a dangerous defect?