Main protective bonds should be not less that half the csa of the earthing conductor and not less than 6.0 unless PME conditions apply, this has been stated as far back as the 15th Ed when I started work. The 15th Ed never quoted a minimum size where PME conditions applied just that the electricity supplier should be consulted it was the 16th Ed that quoted 10.0 as a minimum size for PME systems.
So whilst 10.0 is considered the norm it does not apply to all systems.
There are PME systems out there with main protective bonds sized at 6.0 installed pre 16th Ed and you have no way of knowing if they were correct at the time as they would have been sized to supply company requirements.