The due date you got by counting from the LNMP is wrong
There are several reasons why a due date figured from the LNMP could be wrong. Sometimes women do not remember the date of their LNMP correctly. Sometimes a woman misses her menstruation for another reason, and then gets pregnant later. This woman could really be less pregnant than you thought, so the uterus is smaller than you expect. Or sometimes a woman has a little bleeding after she gets pregnant. If she assumed that was her LNMP, this woman will be one or two months more pregnant than you thought. The uterus will be bigger than you expect.
Remember due dates are not exact. Women often give birth up to 2 or 3 weeks before or after their due date. This is usually safe.
If the due date does not match the size of the uterus at the first visit, make a note. Wait and measure the uterus again in two to four weeks. If the uterus grows about two finger-widths or 1 cm a month, the due date that you got from feeling the top of the uterus is probably correct. The due date you got by counting from the LNMP was probably wrong.