Why are the walls longer on the set than they show in Room Builder?

Problem: I seem to be having a problem with Room Builder. Sets that I create seem to expand slightly when I click "Build Room". 

I've just spent a lot of time building a set for an irregular room, setting precise measurements for each wall. However, when I edit any wall in the green room the length has changed. For example, an 8 ft wall becomes an 8ft 9in wall. Answer:   Here's what's going on -- the Room Builder Rooms are built with INSIDE room measurements. For example, if you build a room that is 8' by 15' in Room Builder (as shown below) 

Then the program creates a room with those INSIDE dimensions, but of course the walls themselves have thickness (6" if you built using feet as a measurement, 20cm if you were using meters). Because they have this thickness, then the walls have be made actually larger than the reported dimension so as to not leave gaps at the corners. 

Here is the same set in Blueprint View after having been built by room builder. I used the on-set measurements and got exactly the right inside dimensions, though if I bring the walls in the Green Room they do show as being 16' and 9' long respectively.