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

I haven’t, but it would be interesting to plug in some numbers and see what it says.

Here’s another big cube/smallish head comparo.......
572, 10.7:1, EZ-295 heads, solid roller, 266/272-110, .672/.645, single plane, 1.560 venturi 4150 carb.
DD shows 664tq/615hp
Real dyno shows 719tq/715hp

That’s 55ft/lbs and 100hp low.

If you were using the data from the DD to plan the build to make over 700hp....... and it’s showing you’re at 615hp...... how much stuff do you change to make the program spit out 700hp+?
That DD2000 program will steer you wrong for that type of build.