Well that's strange. While I have often experienced a cluster of seated tables in an otherwise empty restaurant, 100% of the time that is the restaurant's doing, which wants to seat people together in the interests of wait staff. "Open sections" and "closed sections," as it were.

But I've never, ever seen this happen when people seat themselves. It just seems human nature to leave at least one unit's space between you and another group or person, when the space is available. (In a restaurant, the unit is a table; in other places such as say a classroom or public transportation, the unit is a seat).

People ALWAYS use the "buffer unit." In my experience. Perhaps you smell particularly good or are particularly attractive, and people want to sit next to you.