Reply to "engine dyno vs chassis dyno"

This is very interesting info for me. My friend who has a 71 cuda, 340 4spd, factory hot solid lifter cam (purple cam) holy 750, edlebrock RPM intake and aluminum heads, headers & free flow exhaust.....power tuned on the dyno by the shop, who are outstanding....266hp at the rear wheels.
Same concept on my friend 69 hemi roadrunner, upgrade cam, factory two fours, 2 1/8 headers etc., 4spd. 310hp at the rear wheels.
Ok, whats wrong with this picture, corrective numbers? this well known race shop say's the do no corrective numbers, they basically say who cares, it is what it is at the rear wheels.
Not to get to funky, but it is hard to imagine some of the rear wheel numbers out there, or is it just because our local, well respected, race tune etc. shop is a realistic number? I dunno....My Trans-am made 392hp at the rear tires on this same dyno, pretty much nothing not done to this engine less supercharging. 440cu., long H-beam rods, steel nitrated crank, studed top to bottom, race ported aluminum heads, roller cam, 950 race demon carb, 2" headers NASCAR 3 1/2" exhaust, BG fuel system, 11-1/2:1 compression, o-ringed block on, and on, and on...TKO600 5spd thru 3:73 12 bolt.
All this, and only 392 rwhp.
Now, that said, it without question spanks the living @@$%$#@ out of my friend Hemi road runner, a 70 hemi cuda, 67 GT500 etc.
Oh, 11.13 1/4 mile on street tires. 3655lbs.
392 rwhp.
So maybe someone can see my confusion, the dyno by the way is always kept up to snuff and certified.
I'm also at 2000ft elevation. All shift points were at 5500rpms.

Daniel

P.S. 525rwhp when we hit the Nitrous and spun the tires off the dyno.

P.S.S. Be nice guys, keep the pontiac jokes to a minumum. You won't want to know how many clevelands, 428fe's, 460's and any stroked ford combo I spanked so far.