What name did Turtle call Mr. Eastman? Why?

Ellen Raskin's Newberry-winning novel The Westing Game follows eight pairs of tenants in Sam Westing's apartment complex who must compete to solve a puzzle, the winners of which will receive the full amount of Westing's multi-million dollar inheritance in his will.
Turtle, whose real name is Tabitha-Ruth Wexler, is a 13-year-old stock market prodigy teamed up with Flora Baumbach in the game. Mr. Eastman is one of Sam Westing's aliases that he uses to conceal his true identity.
Turtle encounters Mr. Eastman while trying to solve the mystery of Sam Westing's death, which she believes was staged. She notices a strange pattern in three of Westing's aliases: each represents one of the direction on a map. The only missing direction is east. Turtle takes it upon herself to look up the address in the Westingtown phonebook and pays him a personal visit.
When she asks to see Mr. Eastman, she is told that Eastman was "expecting her." When Turtle walks into the room and takes a look at Julian R. Eastman, she is overcome with a feeling of "panic," perhaps fearing she might be mistaken. As Mr. Eastman smiles at her, Turtle exclaims, "Hi, Sandy!" before proclaiming herself the winner.
This shows that Turtle believes Sam Westing disguised himself as Sandy, Barney, and now Mr. Eastman in order to observe the "game."