As MrSaffron says, the complexity arises when exchanges cover areas across boundaries, or cabinets cover areas across boundaries.
As you mention, postcodes don't stick to boundaries either - but I've seen BDUK projects state that while the summarising maps happen at postcode level, they have to consider individual properties. They don't write a property off simply because they believe, at postcode level, the area is covered.
So, having said that...
The councils are meant to consider the area outside their boundary by a few kilometres and then "negotiate" between the councils to determine which county takes responsibility.
Obviously this works in a reciprocal way, so there is a border zone where properties could be part of either scheme.
When North Yorkshire was the first county getting BDUK approval, and the plans were going through, you could see some exchanges outside the county boundary getting included in Openreach plans... even though they were in a county that hasn't signed a contract yet. I think Snaith exchange (East Yorkshire) is a beneficiary that has gone live, while Bubwith appears to be a "coming soon" beneficiary.
I *think* the distance is 10 kilometres.(as mentioned in the East Yorkshire public consultation document), while the Coventry/Solihull/Warwicks project specifies that the negotiation happens to make sure that every property is included, but only funded once.