Presumably because he hadn't half a clue about fault-finding, which isn't part of either the training or prerequisites for ACS. However he should have been competent at testing that the boiler was effectively sealed and not leaking POCs into the room after he'd done the fan :-|
Prolly still hogging the middle lane. Saw on the gogglebox an age ago about someplace in India where the traffic makes Keystone Cops look like a sedate country spin, but the learner drivers' test course is a completely empty stretch of tarmac about 100 yards long with a slight wiggle partway along. This country's driving test is like that in that learners get a full licence without once having driven on a motorway.
And (to get rather circuitously back to the point (whatever it was :-))) you can get a full ACS and CORGI registration without having a clue about fault-finding problems unless they're directly related to combustion safety.