Why am I a bhai to my male friends? 25F

Hello guys As per to the title just wanted to know few things. Last night we friends(male+female) had a get together, upon some drinks my male friends confessed that I am a bhai to them. To be honest I was happy but a bit sad too..as a tomboy growing up, I became a bit feminine while exiting college. And I thought I was fully feminine but still that bhai in me never left me. I just wanna know from guys what makes guys think that a girl is a bhai to them?