As you can see the approximations it is very large compared with reality.
36 cm (14 1/8") in reality is 35.8775 cm or at least 35.9 cm
70 cm (27 1/2") in reality is 69.85 cm and so on.

And that's why you shouldn't mix the two systems. They simply are not compatible and conversion from one to the other will always remain an approximation. An approximation that's rough in Sweet Home 3D because it's smallest unit unfortunately is 1mm where you really need at least three digits after the decimal point to stay somewhat accurate.
Starting with a new project it should be no problem to choose one of the systems. If you need to create a model with the other system do that in a separate project and export it. Then import it in your main project. Problem solved.