Confused please explain!!

Ashley • Expecting baby girl :)) this will be my princess number 3
My LMP was September 4th so based off that my due date would be June 11th, my cycle was every 28 days usually 3 to 4 days.. Based off my first ultrasound my ob gave me a due date of June 6th!!! So confused does anyone know when I may have concieved ?? I'm just trying to understand this???