WRONG LMP??

SO MY 8 WEEK ULTRASOUND I SHOULD HAVE BEEN 7 WEEKS BUT I AM MEASURING 7 DAYS AHEAD OF MY LMP GIVING ME THE DUE DATE OF APRIL 14th...WITH THAT DUE DATE ITS SAYING MY LMP WAS JULY 8th WHICH IS MAKING ME 8 WEEKS INSTEAD OF 7... BUT MY LMP WAS ACTUALLY THE 15th WHICH IS 7 DAYS LATER.. I KNOW FOR SURE BECAUSE IT WAS HEAVY AND I BLED FOR 5 DAYS...WHY IS MY BABY MEASURING A COUPLE OF DAYS BEFORE MY NORMAL LMP??