As far as I can see, the reason is that some bureacrat within DEFRA has decided that it would be nice and tidy if areas were classified by county.These classifications explained on the DEFRA website. Basically, if there is a 'HIGH RISK' area within a county - either because of recent outbreaks or because blood testing has not been completed, then the whole county has that classification. The whole thing is a complete nonsense. I was telephoned by a farmer in Herefordshire yesterday who has animals in Monmouthshire. Both counties are in the 'HIGH RISK' classification but he has until Monday to bring these animals home because after that the county based classifications come into effect and he will not be able to move them over the border - even thought both counties are in the same risk category! These classifications come into effect on 17th September and are apparently subject to review every week on a Tuesday.
I find it quite beyond reason that someone somewhere can alter the classification of an area just by drawing lines on a piece of paper - one of my biggest criticisms of this whole business is that Form D notices have been left in place long after they cease to mean anything. They just don't seem to care what devastating effect they are having on people's businesses and furthermore, where regulations are so clearly ridiculous, they ask for them to be breached, in my opinion.