How do I have a credit score of 717 if I just started building it? (My fiancée added me on his credit card)

So I just got added on to my fiancees credit card in March, he wanted me to build my credit so he added me on. Mind you I have no credit whatsoever I just started

I have a credit score of 717, which I’m really confused on how that’s possible, my fiancée thought maybe I took on his score but his score is a lot higher than that but I’m still trying to figure out how it got to that score

Maybe somebody can help me understand how this is possible