I think it just varies depending on whether it's an inertia or load cell dyno. Mine did 420/1000 on a mustang in 5th(stock turbo and injectors) and on a 248c in 6th I couldn't get enough load and did 597/1054. I'm pretty sure it's producing more torque than that. When you crunch the numbers they don't jive either way really.
I think we should as a general rule just accept the hp calculators numbers......if we use trap speed it bumps me up a touch into the 600's LOL
My non-existent dyno expertise leads me to agree with Timebomb.