I had 3 u/s first trimester , 6w, 8w, 12w . All with a conception date of around July 2nd-4th . The confusion is I had one period I know for sure on around may 26, next was either implantation or lmp on June 18 , can ultrasounds mistaken my lmp on June 18 when really my lmp was may 26 resulting in 4 weeks off pregnancy

Normally not that much, they can be off at times up to about 2 weeks, but not much more than that.

Next they don't detect your last period, you should know that from personal experience. They merely measure the size of the fetus compared to the average size of fetus at that stage of development. So there is always lots of room for error.