I suppose Steve is right, that the pavers could be on sand or ground with no slab, and that the house is built on a perimeter footing. This is after all the desert. If that is the case, the settling is displacement of the material under the pavers caused by unequal settlement or burrowing animals. It would not be that difficult to pour a concrete floor in place of the pavers if you ever wanted a slab.
Best thing to do would be to get a pre-purchase inspection and know what you are dealing with, and obtain costs of bringing it up to current code or what you want. If there is no slab, at least it would be relatively easy to get to plumbing and other infrastructure normally buried in a slab.