What does Nikon USA model mean? At some online stores they specify USA and they are a bit more then the ones at the same store that do not say USA.
Products specified as USA are intended by the manufacturers to be sold only in the U.S. and the warranties are generally only applicable if the item is bought in the U.S. or shipped to the buyer in the U.S. from a foreign seller. The other items are usually the identical product but are not intended by their manufacturers for sale in the U.S. These are sometimes referred to as Grey Market items. If you purchase such a product and have it shipped to an address in the United States the manufacturer might not honor the warranty if you need service or replacement during the warranty period.
In general, online stores that sell grey market items are viewed as being a little shady and are sometimes known to use deceptive business practices to increase their profits on a sale. There are exceptions, especially stores such as B and H and Adorama, who offer gray market items to their international customers as well as their customers in the United States.
For more on how a shady grey market seller operates, see this answer to another question.
The less expensive, non-USA versions are likely gray market cameras, often exactly the same device in about the same packaging, but imported by someone other than a Nikon-authorized importer and therefore lacking a warrantee.