Why are dynos different? You basically said the answer, a dyno measures torque at rpm. No one is calibrating their dynos based on torque.
Most believe a 212 Predator or a 196 clone makes 6.5hp; this is inaccurate. If you look at the Predator 212 box, it only lists torque, not horsepower. The hp is listed on the website because it was originally duplicated from the Honda GX200(now 5.5hp), but we can disprove it using math. The max torque of the Predator 212 is 8.1ft-lbs at 2500rpm. That is maximum torque, but for argument, let's say it's making 8.1ft-lbs at 3600, which is the most common standard for small engines, or 3800rpms, as is listed as the maximum rpm on the box. 8.1 x 3600 / 5252 = 5.55hp or 8.1 x 3800 / 5252 = 5.86hp. For the Predator 212 to make 6.5hp, it would need to make 9.48ft-lb at 3600 or 8.98ft-lbs at 3800rpm which is higher than its listed max torque, a major contradiction.
Interestingly, Honda, which lists 5.5hp at 3600rpms, also makes 9.1ft-lb at 2500rpms. How many engine builders can say that the Predator 212 makes 8.1ft-lbs at 2500rpms while 6.5hp at 3600rpms? Another reason the torque number would be more accurate than the advertised horsepower number goes back to a class-action lawsuit with Briggs in the early 2000s, where the horsepower numbers were taken out of context or manipulated to sell engines. Torque is now the legal representation of the engine's output. Those lawsuits are why the Honda is listed at 5.5hp, not 6.5hp. Harbor Freight, to my knowledge, doesn't give an SAE number, so you don't know what correction factors or test procedures it is using, but Honda does. You can get really messy with correction factors(observed vs. corrected), net vs. gross power, test conditions, etc, which are rarely if ever, provided along with the dyno sheet.
If the question is serious, which of the two engines is faster, the engines have to be comparable, and the only way you can do that is by having them on the same dyno. Regardless of the numbers, we are overlaying them to look at power under the curve and how that power is applied on the kart(gear ratio/tire size).