Wonder why so many house garage floors have cracks? Rarely do I see the soil under the garage floor get compacted after it is put back inside the foundation walls. To do this manually is time consuming and many excavator operators don’t bother changing the bucket of the excavator to a compactor attachment, if they even have one. So the brand new concrete floor gets poured over loose disturbed soil that will eventually settle. This settlement of the soil causes shifting which may cause the concrete floor to crack. Structurally this is not so important and it will still support the weight of your vehicle. It could be considered a cosmetic issue which is why so many builders let it happen. A different issue and possibly more harmful would be backfilling with aggregate containing Pyrite. Pyrite is affected by humidity and oxygen and when combined it can produce gypsum crystalization. This gypsum crystalization is what causes the floor to swell and crack. This swelling could (although rarely) cause serious structural foundation problems.