I tried using a meter just like that one a short time ago, it had been calibrated less than a year previously. Results were 3k 91, 3.5k 93, 4k 96, and 4,5k 98 dba. this is with standard original S1 exhaust system, 4into 2 into 1 manifold and shiney tin box. Pinto with 285 cam. I think that the results were very kind to my car. So don't be too suprised if the official readings are worse than your unofficial ones.
Peter.