I have seen this many times where the builder has not tamped the garage floor or watered the sand or gravel to prevent this. Time is money to them so they just get it done. Same with a drive way, if you pour it with in the first year you are going to have settling. This is why you have the void. If you tap the floor and drive way you will be able to tell all the hollow spots. You can get slab jackers in to fill that void. They will mix up a slurry and pump it under the slab and fill.