The Great Gatsby, somebody

I'd like to know why in the chapter Six of The Great Gatsby, by F. Scott Fitzgerald, the narrator says that "somebody brought Tom Buchanan in for a drink" if, in fact, Tom Buchanan was accompanied by two people. I thought that "somebody" was used for one person only. Could it be used for several people? I have observed that in the fist page of the chapter Seven, in order to explain why he has fired all his servants and "replaced them with half a dozen others", Gatsby says : "I wanted somebody who wouldn't gossip", because "Daisy comes over quite often". In this case, too, Gatsby seems to be speaking of several people, his servants, in making use of the term "somebody". So it could be an idiomatic expression particular to Fitzgerald.

"I hadn’t been there two minutes when somebody brought Tom Buchanan in for a drink. I was startled, naturally, but the really surprising thing was that it hadn’t happened before.

They were a party of three on horseback—Tom and a man named Sloane and a pretty woman in a brown riding habit who had been there previously."

Thank you.