The length may vary because the sellers aren't consistent in the spot at the top of the coat where they start the measurement. The length shouldn't vary more than an inch or so.
The length of a peacoat is properly measured from the seam where the collar joins the body of the coat, down to the hem of the coat. If you ask them to give you this measurement, they should be fairly standard. The more critical measurement is the sleeves. They should be measured from the seam where the sleeve joins the shoulder, down to the cuff in a straight line, not following the bend of the elbow. A tailor will measure from the middle of the neck, just below the collar, down to the cuff, following the bend of the elbow. We aren't tailors.