2 different dynos,2 very different numbers.why?

You have to get things down to basics. ALL THAT a dyno "is" when you take off the hood, the stickers, the fancy software, and all the rest, is a TORQUE MEASURING DEVICE. ....PERIOD..... HP is computed from that torque reading + RPM, and I'm sure you realize, that getting "halfway accurate" RPM readings are not difficult.

SO THE TORQUE MEASURING DEVICE has to be accurate, and if it ain't,...........well..............