Mainline vs Dynapack hp numbers

Just out of curiosity, what are the differences between the readings of Mainline and Dynapack if you measure the same car?

Significant would be the easiest way of describing it. From my own testing it seems that the mainline reads about 15-18% lower than Dynapack. This isn't much good for internet bragging rights but if you're treating the dyno as a tuning tool then it doesn't really matter what the reading is as long as it's repeatable.

I have this descusion all the time because i have a DynoDynamic and my competitors have Dynojet dyno and my DD also real like 15% lower than Dynojet