Part of what makes le Carre's fiction appealing is its emphasis on human nature. Le Carre's novels are not about great armies of spies that clash in the night, deciding the fate of all humanity. Instead, they are stories of individual people, whose personalities shape what they do. Central in The Russia House is Barley's personality, not the characteristics of his government, or his spy agency, or the Western world.
Ned recognizes this, which is why he knows well ahead of time what Barley really intends to do. Ned's colleagues do not realize Barley's intentions; they naively think their training has somehow fashioned Barley into the faithful drudge that he could never truly be.
Thus, they are surprised by his betrayal.