The supporting soil will inevitably settle to some degree
It is common for the ground between the stiffening beams to settle over time. The settlement is usually minor, but one to two inches is common. This is true even if the soil was well compacted. For houses founded on soil that was not properly compacted, the settlement is likely to be higher.
Local floor slopes
When the between the stiffening beams settles, the 4-inch thick concrete loses ground support. Some degree of sagging is inevitable. This problem is more severe with post-tensioned slabs because they have no bonded reinforcement: commonly known as rebar. Conventionally reinforced slab-on-ground foundations have some resistance to the sagging. In older slab-on-ground foundations, you sometimes may feel as if you are walking up and then down all across the slab.
These local floor slopes are a nuisance, but they are not unsafe and, in fact, are expected.
Floor slopes due to overall deflection
When the foundation is supported by expansive soils, the clay soil will absorb moisture and swell. In most cases, the majority of the swelling will be in the middle area of the slab. This swelling begins soon after the foundation concrete is placed. It may continue for around 7-years. The swelling clay pushes upward with a force of as much as 5000 psf. The slab is concrete and resists the swelling clays so that the middle area may be relatively flat, but the slab surface near the perimeter will necessarily slope downward.
Floor slopes due to deflection plus tilt
In some cases, the foundation will tilt downward toward, say, the perimeter and the foundation will also bend downward in the same direction. The result is that the floor slopes downward in the same direction. In other words, the two slopes can add together.
On the other hand, the slopes can cancel each other out so that the floor seems level on one side and severely sloped on the other side.