The most common misconceptions about slab-on-ground foundations
Slab foundations are constructed level
This is not even remotely true. The American Concrete Institute (ACI) publishes levelness tolerances for slab-on-ground foundations. Although this is a simplification, two points anywhere on the slab can be 1.5 inches different in elevation and be in compliance with the ACI construction tolerances. Two points 10 feet apart are allowed to be as much as 1.25 inches different in elevation.
To make matters worse, the ACI says that their construction tolerances are about as good as you can expect.
If a house on a slab foundation shows signs of foundation movement such as cracks in the brick veneer or drywall, the foundation needs to be repaired
This is wrong on numerous levels.
If a slab foundation is out of level by some amount, the foundation should be releveled
Here is a direct quote from an Army Corps of Engineers manual titled Foundations In Expansive Soils:
Most slab foundations that experience some distress
are not damaged sufficiently to warrant repairs.
Notice that there is no mention of levelness or even elevation measurements of any kind. The Army Corps of Engineers uses damage due to foundation movement, not levelness, as a performance/repair criteria.
If repairs are done well, the repaired foundation should be level and stable
Here is a direct quote from an Army Corps of Engineers manual titled