Measuring your fundal height isn't necessary and it can vary depending on the position of the baby, the amount of fluid or even not having the tape in the same place each time or being measured by someone different can give you a different measurement. Many care providers don't even measure it anymore as itsn't an indicator of anything other than the height of your uterus. I wouldn't be concerned at all about it. Ultrasound isn't even an accurate measurement of baby's size either.