I get this comment a lot, casually, from parents of boys. Equally I hear things like "girls are complicated" or "girls are difficult".
My personal experience (I have both boys and a girl) is that they are pretty similar.
I also believe that there is no fundamental difference between girls and boys other than how they are shaped by society. Assuming you agree - and am happy to hear those who don't agree too obv - what kind of response would you make to those throwaway comments to gently challenge the idea that girls and boys are fundamentally different?