A roadworthy is not a full inspection for registration. As already said, noise testing is not a requirement, nor is meeting all relevant ADR's. Basically, all they do is check that everything works and there is no structural rust.
Also, just because a copper has cited the car for being too noisy, doesn't necessarily mean it actually is, only in the copper's opinion.
One would hope that there is consistency in all aspects of vehicle inspections and checks, but because humans are involved, opinions vary greatly so there will always be differing results. As an example, I have had a car inspected many years ago at two different places, only because the first place closed after the initial inspection, I had rectified the couple of things noted on the first inspection, but since the second guy didn't do the first check, he went over everything again and picked a couple of extra things that weren't noted on the first one.