*Before you read any of this as fact, please read note on bottom!*

Its kind of hard to explain because i do not exactly know how it works but the SPL that it displays does not mean that is the actual output SPL you'll get on say... a meter or something.

basically all that graph is showing you is how 'efficient' that box will be to the speaker itself and in reproducing sound as a result. Obviously, the flatter the graph line, the better the response will be since it won't be so focused/specialized to one specific frequency and thus sounding terrible in real life.

You have to be aware that even though the program can show you the ideal graphs and build specifications, they are contained in an infinite universe and perfect world. We don't live in a perfect world and you're not perfect, there will be mistakes. But trying to get your box and setup made and put in the best way possible will help overcome that.

Good luck.

*None of this is to be regarded as fact, i try to explain things in the way that i understand them and the ways that they were explained to me. If i am incorrect in any of my above statements, by all means please correct me in a non-insulting way or in such a form to intentionally cause me embarrassment.*