1/4" is 1/4 " .... a good way to check if you are getting the right measurement is to sew three 1-1/2" strips together and re-measure. If it's 3-1/2", then you've got your 1/4" bang on.
If not, you're not doing 1/4"!!
Sometimes it's easier to move your needle position to get you that precision, than to change how you sew, especially if you're using a 1/4" foot.