Same as in English, we have to be careful about saying 'I know' sometimes even when you 'knew already' you should not always readily say so - so I think 明白了 is a way to be polite, even though it is not strictly correct. (其实已经知道了,但是还该说‘明白了’ To be polite in English you would say 'ah, I see' whilst under your breath muttering 'come on, I always knew that!'