As long as the dyno is calibrated with weights it should be accurate. The other thing is the dyno will read the HP and torque when you pull the engine, but most have software to adjust to an SAE standard, so you can ask the dyno guy to give you the real numbers not the adjusted ones. I got an older cheap land and sea water brake dyno and I calibrate it with 200 lbs of weight on the torque arm. I was told the heaver weight you use the more accurate the calibration. I just did a GM 427 crate engine for a guy, and GM said it should make 480 HP and 490 ft. lbs. On my dyno it made 483 HP and 513 ft. lbs I had a pretty good set of 1 7/8 dyno headers and I push the exhaust though 5 inch exhaust and two big truck mufflers. So I am thinking that is where the extra torque came from. The peak numbers were within 100 rpm of the factory specs as well. But a lot of things like oil temp, water temp, humidity will all affect pulls even back-to-back. And the dyno program to adjust to SAE numbers only does so good. If you pull an engine on a 70 degree low humidity day compared to a 95 degree 70% humidity you will get lower numbers on the hot day corrected or not.