Your deck is a certain size, and your dimensions are probably from the wall to the outside of the deck, not from the deck edge to deck edge. Typically the deck will stay the size you drew it, and the dimensions will position it.
The deck joists will follow the reference edge, so if you slope a reference edge the joists will slope.
You can make sure you dimension the actual deck edges by clicking an extension on the deck not the wall and work from there, OR you can use set distance between to edit the size of the deck (again, make sure you click on the deck, not the wall, OR you can use ADJUST to stretch the deck edges to where you want them. There are multiple ways of doing what you need - Softplan ONLY does what you ask it to, the problem is, sometimes you haven't asked what you think you asked.
I've been working on CAD systems since 1983, and used many systems. Softplan is without a doubt the most intuitive I've ever worked with, but then intuition is different in everyone.