-When you going back to kindergarten, Otis?" some one had asked.
-"Me? Day Bernice gets her hair bobbed."
-"Then your education's over," said Marjorie quickly. "That's only a bluff of hers. I should think you'd have realized."
Source: Bernice Bobs Her Hair by F. Scott Fitzgerald
I don't understand why the girl used "should think you'd have done" in the example. I understand that 'You would have realized' belongs to the third conditional. But why was it used with "should think".