Very true! Plus a lot of times, especially in the 60s and early 70s, there was a lot of lying going on from Detroit in terms of actual horsepower. They would often intentionally derate the true values and make them seem weaker to defeat insurance companies that were either going to rake you across the coals or flat out deny you coverage.
One of the most infamous examples as Ford issuing out the 1968 GT-500KR and claiming it was 355Hp when in reality, it was well beyond the 400Hp mark. Insurance companies kind of set a limit to under 400hp for production cars before they designated them as race vehicles, so then nearly every Detroit company began lying their asses off about what they really had lol
But another factor was when the cars switched out from leaded to unleaded which immediately began devaluing horsepower for real. Leaded gas had much more compression and overnight, that was lost on every car. Some cases showed as much as a 50Hp reduction from leaded to unleaded in some vehicles, which would've been extremely frustrating to those that had just bought a car with high horsepower. The fuel additives like octane boost or lead substitute, supposedly helped get them back over the slump
