Why are women so snobby and catty to one another?

I have had alot of experience with this. I hear of girls that claim not to like me whom I’ve never even met. My mother always taught me to smile and be nice to everyone. She also taught me to never forget my manners. Maybe I was born into the wrong time period or have women always been like this? Does anyone have any experiences? Here is an example of mine. I go to the gym walk past a women(stranger) smile and she just kind of looks away or gives me a funny look.