I've seen A LOT of posts about making HP with a 318-So why is it so ???

I bought a desktop dyno about 20 years ago.
I was doing a lot of dyno testing back then, and already had a pile of dyno sheets to compare with.
If you modeled something like a SBC 383 with high CR, a big roller cam, nice intake, headers, carb, good heads....... you could end up with the numbers somewhat close-ish.
But any kind of BB Mopar with std port heads and a big cam....... the numbers were way off.
On my old version, the real numbers were usually quite a bit better than the DD numbers for the std port bracket race type 440.

I had modeled two motors that were going on the dyno, just to see what it spit out for numbers.
A 446 and a 493.
Both street/strip builds with RPM heads.
Both got very similar solid cams, same intake, carb size, same CR, etc.
The DD was showing the 493 would make about 40-45hp less than the 446.
On the real dyno, the 493 made about 30-35hp more than the 446.

One of my customers has DD, along with two other, quite a bit more expensive engine sims, that have way more intricate inputs....... and they’re not all that close on many builds either.