I'm not 100% sure to understand your issue, but it seems to happen because you didn't get that walls length are measured in their middle, not along their sides. When a wall is bound to another one to form a corner, this can can lead to a longer or shorter length along wall sides. Therefore, it's not a problem of precision and you should take take into account the wall thickness.
The video How to design rooms from their inner dimensions might help you too.