Ferring IVF wheel

Does anyone else use the Ferring IVF wheel for pregnancy dates? I’m a little confused because it gave me the correct due date, and it says I’m 6w4d when all of the other apps say I’m 6w5d. So which is it? I put the same due date into all of the apps based on the due date that was calculated by the Ferring wheel (Glow, what to expect, pregnancy +, Ovia, etc). My dr said I should be 6w5d based on my transfer date, so why is the Ferring app different?