Most Helpful Guy
I used to do it calling my girlfriend by my female best friend name.. and sometimes I call her by her sister name. Most time I stop myself before it comes out to something.
I believe it's because of how we store information, my brain thinks "this is a girl I spend time with, the default is this person as I spent lots of time with them" after being wth his girlfriend for years you might find he starts calling you her name
It's like with pets, I sometimes call one by another's name.. or my mum when I was growing up would some times try and call one of us and get it wrong every time until she said them all.
Most Helpful Girl
It just means y'all are close. I mix up my male coworker and my boyfriend's names sometimes, but all 3 of us also hang out after work and stuff often enough that I mix them up in stories even though they're totally different.