2 different dynos,2 very different numbers.why?

For the last 44 years I’ve been racing cars that have run in the 8’s, 9’s, and 10’s and the first question people ask is “how must horsepower does it have.” I tell them I honestly don’t know as I’ve never had one on a dyno. The 572 I’m building now won’t see one either. I’m not a show car type of guy so all I care about is the ET numbers.