383 for a 67 cuda

Hey all,
If I'm not mistaken a 1970 383 hp engine out cuda had a factory rating of 335 hp in bone stock condition.
In the day we were able to do a good tune up 3 way value job, some mild porting, a purple cam, headers and we good for almost 400 hp.
So why would a stoked 383 with the same work not produce at least 500 hp?
I just built a mild 360 that is approx. 400 hp.

Rick