I disagree Troopy, I have also spent many years in construction...

I disagree Troopy, I have also spent many years in construction and a window opening is always a few MM bigger than needed and window/doors are packed to fit, very rarely did I see any openings cut to fit windows and doors. Slabs also can be accurate but takes a bit more time, they give concreters about 5mm tolerance because they are concreters, laser levels have the ability to be mm accurate but the set up needs to be mm accurate as well and that takes time.
Had a boilermaker friend doing setup for a concreter for awhile and the slabs were near perfectly square, Even the boss was in awe, Bit of accuracy just takes a bit of time, if it means spending another few hours getting slab right to get the Blocks laid in a day then thats what you do. Have also seen some slabs with no pooling after rain, but that was more good luck than good management, so it is possible.