Why does Jaggers call Wemmick the "most cunning imposter in all of London"?

In chapter 51 of Great Expectations Pip wants Jaggers to be more forthcoming about the identity of his benefactor, not just for his own sake, but Estella's. Pip is sure that he already knows the truth, but wants to hear it from the horse's mouth, as it were. But initially it seems that Jaggers is reluctant to divulge the full story, so Pip turns to Wemmick, who's also present, to intercede on his behalf.
In making his appeal, Pip refers to Wemmick's kind heart, and also to his father, the implication being that he's a good son who dotes on this senile old man. Upon hearing this, Jaggers is rather taken aback. This isn't the Wemmick he's come to know; the Wemmick who works in his office; the Wemmick who can so often be such a calculating individual in his business dealings. That's when Jaggers jokingly refers to Wemmick as the most cunning impostor in all London. It's as if he's been leading a double life all these years.