I would have thought that it was blindingly obvious why the map only shows the exclusion zone extending to the Welsh/English border.
The Welsh Government and in particular, the Chief Veterinary Officer only have control of matters that affect Wales.
I would expect that the CVO’s English counterpart has been informed and it would be up to him/her to complete the circle for the English section of the affected area.
Mind you, I thought about the reasons why before passing comment or making erroneous assumptions.