This is because of the way that the Police Reform and Social Responsibility Act 2011 was written, before mayors of combined authorities (like Greater Manchester) were introduced!

Boris Johnson was able to serve as both Mayor of London and an MP because the Mayor of London is not considered to exercise PCC functions under the Police Reform and Social Responsibility Act 2011 and so that restriction did not apply, instead they are 'The Mayor's Office for Policing and Crime'.

Clause 16 will also apply to 'double hatting' as both a Mayor of a strategic authority and as a MSP, MS or MLA.

And the reason why section 67 is understood to apply to Andy Burnham is because of article 3 of The Greater Manchester Combined Authority (Transfer of Police and Crime Commissioner Functions to the Mayor) Order 2017, which essentially says that the Mayor is to be treated as a PCC.

You are right: mayors with PCC functions cannot also be MPs (see section 67 of the Police Reform and Social Responsibility Act 2011).

Clause 16 of the English Devolution and Community Empowerment Bill will also prevent all mayors and strategic authorities 'double hatting' as MPs.

I would expect no less!

I love the synchronised clerks!