Houses with slabs are always suspect because contractors don't always remove all the topsoil or compact the soil enough. Driving a grader over the ground to get a level pad doesn't necessarily compact the ground as much as needed.
Road builders use a sheepsfoot pulled over the ground to compact it as needed and then compaction studies are done.
Houses with basements are usually dug deep enough to get into compacted subsoil but depending upon the soil type and location, that may not be enough either.
Different areas develop different standards for their soil types, size and depth of footings, etc., but all builders may not follow the maximums.