I have an existing 3 way in my house that is wired like this:

Staircase..
My feed is in the 1st floor fixture with a 3 wire going down to the 1st floor switch.
3 wire from 1st floor switch to 2nd floor switch.. and a 2 wire from the 2nd floor switch to the second floor fixture?? I removed switches to remodel before realizing how it was wired.. An explanation would be appreciated.