I have read a couple articles now from Thrifty Traveler where occasionally folks are finding higher airfare for a person traveling alone than for a reservation for 2 plus people. Yesterday, the airline was Delta, today Thrifty Traveler is reporting that this is happening with other airlines (United and American). The airlines are not responding to requests for comment. It is not for all tickets, it is seeming to occur sporadically. Sounds like the algorithm might put travelers in different fare classes based on the number of travelers.
In the past, occasionally when I've searched for one person and then expanded the search for more travelers, the airfare goes up. This happens when there are not enough spots in a particular fare class for the group. (In that case, I usually book individual tickets to get at least part of the group at a lower fare)
One comment made is that "its just like a single supplement for a tour". Well, not really, because its not like the single traveler gets the whole row of seats.