Why are some guys nice to women but rude to men, I'm a man and I just don't understand

I've seen this happen so many times in my life I've had professors or teachers who would be absolutely rude to men, but be absolutely nice to women. For example in my college, my friend had copied the calculations of a girl, and showed it to the professor for correction ( not an exam ), and he got blasted for it and was told it was wrong, and when the girl went up, it was all perfectly fine? She got full marks for it.. And in my workplace too, I've seen countless colleagues be absolutely nice and pleasant to women but not to men. Recently we had to work with IT support to get our laptops fixed, and during the course of it, he was rude to me and my teammate, and even took his sweet time by playing carrom board with his teammates, but when a woman needed his help he left it and came to her help. LIKE just why. Just why. You can simply on them but why can't they just have basic decency to others!??