I agree with allan.

Fundal height can change between practitioners technique, where baby is laying, if engaged etc etc.

I have always measured 3-4 weeks less than i was (ie 36 weeks measured 31-32 etc).
Dr's thought i might have low fluid and had a scan and everything is fine. Bub is just petite :P

They also say that it is very inaccurate after 36 weeks although the midwifes at my hospital seems to still do it...

I think the best thing to use the fundal height is to see that there is a increase each week rather than how it correalated with your dates.