It looks like there's some discrepancy between how the triangle is marked and a normal ruler, as your cutting board shows you've cut 3 3/4. Just for interest, have you laid your triangle straight onto your board and lined up, say, the 3" marks, and see where the triangle measurments actually start from?
However, as others have already said, it won't make any difference to your quilt, just as long as you cut your triangles all the same size it will come together just fine. Don't worry about it and keep cutting and sewing :)