The character of Julia Hoffman changed quite a bit over the course of the show. Early JH was rather manipulative and self-serving, she was on a quest and woe to anyone who got in her way. I don't think she cared much about lying in pursuit of her goal--which in her
fictional mind would lead to an astounding medical discovery.
Later Julia had a serious conscious and cared about the Collins and their friends. She didn't like using or lying to them.
It happens a lot..I barely recognize the Monica of early
Friends episodes with the neurotic mess of the last episodes. .