The "default" is a compromise: not quite US-only, but not the entire world, either.
In addition to the domestic or North American items, eBay will include some international items that meet certain requirements regarding shipping time, possibly also involving shipping method or shipping cost. The longer the shipping time and higher the shipping cost, the less likely an international item is to appear in a "default" Item Location search.
Try a search and then select various location options to get a feel for how many items you see in each type of search, and which items are left out of the default search.
