Noise laws are very tricky as certain cars are allowed to be louder than others. For instance, you are not allowed to modify your Corsa to be as loud as a Lamborghini.
The Lamborghini that was deemed to be too loud may well have been completely standard and legal according to all the relevant laws (Construction and Use, etc.)
There are also many factors that contribute to high (or low) readings. It will be interesting to see what happens if these fines are challenged in the courts.