I had the test done 6 years ago, so things may have changed. There are a number of reasons., aside from mechanical changes.
1 - Equipment, calibration and the MP operator - I had to do 3 passes at WOT. The MP averaged those 3 passes to come up with a final DB number. So there is variability. Each pass was between 2 - 5 DB different.
2 - Proximity to MP boat - For the 3 WOT passes, I was asked to be 25 off of the MP boats transom (scared the &^%& out of me at 70 mph, and a bit dangerous I thought). A few feet either way can mean a difference difference between pass / fail.
3 - Weather - My first test was postponed due to fog. The MP told me that humidity, wind, etc can effect the result.
You know, I moved to south FL 6 years ago. Amazing that this debate is still going strong. Here, no 150 foot rule, most places, no speed limits. Boats of every size, shape and type. Everyone seems to get along (sometimes tolerate) and enjoy boating. There are boneheads of course but's that's another topic
There are a lot of things I miss about living in NH, but this isn't one of them.