Was she an experienced midwife?
I found the measuring to be a bit variable. Certainly +/-2cm didn't bother them but you have quite a big difference.
At the hospital I was told by a doctor exactly what she was doing - that she put the tape measure marked side down on my tummy so she couldn't cheat then checked it. The midwife at my local clinic didn't do it that way, she looked at it and when it wasn't what she expected she just did it again and got it a bit nearer what was expected.
To answer your question, I never had such a big difference.

my doc finds the top of the uterus and puts her finger there, then runs the tape to the top of my pubic bone so that she can't cheat, like AbiW said. also, remember, it's not an exact science.

She was a fairly newly qualified midwife but she had a very experienced midwife with her running the clinic (they were both doing my appointment).
She found the top of my uterus and put the tape there from 0 and ran it down to my pubic bone to measure and 35cm was where it hit my pubic bone. She did it 3 or 4 times to check and the experienced midwife was observing the whole time.

I'm 30 weeks now and at 21 weeks my fundal height measured 27 cm. At the time I was with a midwife (I've since switched to an OB) and questioned her about it. She wasn't concerned and checked my records and I've been measuring large all along. Seems my little boy likes to grow up into my abdominal cavity more than he does out. She explained that it's just the way I'm carrying and nothing to worry about.

When I switched to the OB, who still does the FH measurements, but never tells me and I never ask, she has not commented at all. She did explain that my ultrasound results (I'm 35, so I've had a few because of my age) are much more accurate and she relies on those more.

It could be something as simple as the way you are carrying If you are still concerned at your 31 week appointment, go for the scan to put your mind at ease since that's more accurate.