Why is there a difference on advice for subdivision of corridors  In building regulations it states "Where a corridor exceeding 12 m in length connects two or more storey
exits, it should be subdivided by self­closing fire doors". However in the local government fire risk assessment guides it states a corridor that exceeds 30m in length should be subdivided. 

Why is there such a difference and which is correct?

Building Regulations only apply to new builds, the DCLG guides replicate the approach used to good effect for over half a century and so as not to penalise existing buildings.

The current approach offers enhanced safety, but the old approach still provides a reasonable level of safety in most situations.