door frames - recesses

simple questions should be easiest to answer!
I am fitting a wooden, external door frame to a brick wall. (Not bricked yet). Can anyone tell me why the outside walls of the frame, (right down the middle, both sides), are recessed by about 5mm, about 35-40 mm wide? I thought to take sealant.
Also top and bottom are recessed similarly, not exactly the same. Must be standard.
(The factory seller told me 'ask your builder, I'm busy).
I'm the classic apprentice - new to any kind of building, but having fun.
thanks fellows.


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