Why are some of the characters unsure about who their hosts are?

Captain Philip Lombard is not sure why he has been assigned to the island, other than to hold himself "at the disposal of a client." Miss Emily Brent, an elderly woman, has been invited by letter by someone she met years ago at a guesthouse. General Macarthur, retired from service, was invited by "a man named Owen" to "chat about old times" and Dr. Armstrong was asked by letter to treat Mrs. Owen's medical condition. Dashing young Tony Marston also received a letter from the Owens inviting him to the island. None of them, however, are very clear about who the Owens are. While Mr. Blore travels by train to the island, he writes down the names of the seven people we have just met along with two servants, Mr. and Mrs. Rogers, and decides to pretend to be a Mr. Davis. As Fred Narracott, a local sailor, takes them all to Indian Island by boat, Vera notes "there was something sinister" about it and "shivered faintly."