Pretty nifty measurement/tool there.
Unibox uses three presets: Minor Leaks, Minimal Leaks, No Leaks. These correspond to a Ql of 20, 30, and 50 respectively.
WinISD defaulted to a Ql of 10 for me- that's seems like a pretty leaky assumption!
Sounds like if you know your box will/has no leaks, go with a Ql of 50 for better simulation.
Also, right next to Ql is Qa, or the Absorption factor. Give this article a run over for comparisons: http://www.data-bass.com/data?page=content&id=79
WinISD defaults to 100 for me. Unibox has four presets: Heavy Fill, Walls Covered, Minimal Fill, No Fill. These correspond to a Qa of 5, 20, 80, 120.
So WinISD's default assumption is a pretty leaky box with minimal to little fill.
Here's the difference between default WinISD parameters, and Heavy Fill (Qa of 5) with No Leaks (Ql of 50) for a SI-18 in 4cf with 1000w: