It's not just the political system - they state the ranking is for the whole public sector - so any one that works for federal or state governments, like police,customs officials, park rangers etc.
Which is for me what makes the UK scoring as highly as it has so very surprising. I also work in the public sector (currently) and have seen first hand how very corrupt things are on an everyday basis. It doesn’t seem that a person can be recruited or a contract can be awarded without some degree of favouritism, influence or bias, even at a very basic level. I’ve worked for both national government and local government and quite honestly, I’ve been appalled by my experiences.