While we may like to think of the Western intelligence agencies as all knowing for large periods throughout the Cold War they were essentially blind as to the inner working / thinking of the USSR. The occasional defectors or agents who could bring understanding are significant because of their scarcity.

The West had many reasons to believe the USSR was stronger than it was, hawks on the right wanted to believe it, fellow travellers on the left wanted to believe it, the Russian political system wanted to believe it (and lied to itself repeatedly).