Just like they don’t make sure the cars are properly up to temperatureI have a TT mk7 fitted to my TF, it failed emissions first time out, partly from a cracked joint at the flexi section but also as a result of a peculiarity sometimes with performance exhaust units. That is a “lazy port”. In essence the first port creates a Venturi effect and draws in air from the rear of the car, causing a back flush of oxygen enough to send your lambdas off the scale!! The answer is to cover that first port, put the sensor probe in the other and Bobs your uncle correct reading, car passes emissions!!
Why so called professional mot test “engineers” are unaware of this problem is completely beyond me, but it saved my bacon