The dimensions are not continuous, meaning they start and stop from different locations. I see this alot from people who draw there own plans or from those just starting out. It is easier to layout walls if you are pulling a tape and can just snap lines without adding 3.5", or whatever unknown...
I'm surprised you didn't run into issues with the way the interior walls were dimensioned on the plans. You don't pull dimensions from one side of the wall and then the other. Looks like it is working out as you have walls up.
Wondering if you've considered recycled materials to save money...