the size will vary a lot as you go pregnancy and not always stay at the same amount above or below the number of weeks.
The midwife will not usually be concerned about a difference of 2cms unless there is any other cause for concern. I have a history of large baby and so when i started to measure more than my weeks I was sent for further scans.
The babies position changes and this can also effect the uterus height as can the babys head engaging
Try not to be concerned. The midwife will have seen this in most pregnancies.