Merchant
I have often noticed this at Aberdeen - doing an out & back from LHR day trip buying and changing nothing
As you say i think they have the gain turned up to keep them busy - the same setting at LHR would have queues back to Slough
Even at LHR I also believe they only allow a certain number of people to pass through each gate without checking someone/anyone - maybe a fixed number, maybe a random STOP generator
If you're in a long queue you often see maybe 5 passengers waltz through and then someone being stopped