For a simple question: a simple answer
Slab foundations are designed to resist bending. They are not designed to resist tilting. Tilt is what causes most high elevation differentials.
An example for your consideration
A 1% tilt can result in a difference in elevation as 7.2 inches. Consider a slab foundation that is 60-feet from side to side. 60-feet is 720 inches. The most restrictive published number we have for tilt is 1%. One percent of 720 inches is 7.2 inches. One percent is where most people begin to notice the slope. Beyond that, 1% has no structural significance at all. Yet, we have an elevation difference of 7.2-inches.
Consider an extension to accommodate tilt in 2-directions
Our first example was a 1% tilt in one direction only. Now consider that the foundation tilts 1% from front to rear in addition to tilting 1% from side to side. Let’s say that the distance front to rear is 30-feet. That comes to 360-inches for a 1% tilt from front to back. That elevation differential adds to the previous elevation difference of 7.2 inches plus 3.6 inches for a total elevation difference of 10.8-inches.
Now let’s consider bending aka deflection
Engineers generally want beams to bend to be no more than an inch for every 30-feet of the span. From side to side in our example is 60-feet. That creates another 2-inches of differential to be added. Of course, the foundation can also bend from the front to the rear adding still another inch to the differential. Now it is up to 13.8 inches. You read that right, 13.8 inches out if level. I would venture that there is not a single person in Southeast Texas who has ever seen a house foundation this far out of level.
The main driving force creating large elevation differences is tilt, which causes only very minor distress in the house, and not bending which does create distress and damage to the house.
Stiff slab foundations mean low bending, less distress and more tilt with large elevation differences
Stiff foundations result in less distress and less damage, but there is a cost: higher elevation differences.
In situations where the slab surface is less level than normal, the elevation measurements should be used to estimate the bending and tilt in both directions: front to rear and side to side.