it's all about micrphone placement.

2 MAJOR factors here (thee are many more)

1) how far is the mic from the track - if its further from the car it will read quieter
2) at what point of the tracck is it - at the end of a straight where people are on full chat it'll be louder than just after a corner for example.

the envirnment the mics are placed at on every circuit is different so you cannot directly compare them. the only thing you can compare are the static noise tests as theyre measured at the exhaust tip