Modern HP ratings are flywheel numbers, NOT rear wheel numbers. I believe I read just a few years ago manufactures were forced / agreed to assimilate on how they rate their vehicles' power, to obtain more accuracy across the board. Same with fuel mileage claims as well.

Rear Wheel numbers are a fairly new (last 10+ years or so) way to easily gauge how much power an engine is putting out after the drive train has absorbed its toll. Manual trans and lighter duty rear-ends will absorb less than a stingy auto trans with a converter. I'd assume a Dana 60 takes more “oomph” to turn than a 8.75 too. Also torque / stall converters multiply torque, so real wheel torque numbers will be inflated some with an auto trans.

There are three (maybe four) popular chassis dyno systems, with Dynojet being the most popular (industry standard). The numbers can be listed corrected and non-corrected. Non-corrected (SAE) numbers are what the car actually put out that day. Corrected numbers take into consideration other factors such as weather and issue performance number based on a correction factor, so that someone in Death Valley California in July can compare their numbers with someone is Denver Colorado in January.