For a change it is NOT MA [PLCs] fault. NI/IOM etc are still covered by "Special Branch". Hence you "depart" from a "security" gate [Normally G3] and then [on arrival] get "bussed" to another [secure] gate
Why does still happen at Manchester. It doesn't happen at Liverpool (which has a lot more passengers travelling to Isle of Man, Belfast and Londonderry than Manchester), Glasgow, Luton or Stansted.