The initial chart they use to plot your fundal height on is personal (ish) to you. It's forecasted based on your weight at your booking in appointment so that of a 10st woman would be different to that of a 15st woman.
Also, they'll just plot it as your starting point and when you get measured next that's when they'll make a comparison as to if you're growing "correctly"
Got the record, I measured at 28 weeks, then the following appointment I hadn't grown enough so went for a scan, the appointment after that I'd grown too much and got sent for a scan ... baby arrived at 39 weeks exactly weighing 7lb 12oz. She was forecasted to be 8lb 1oz at 40 weeks so I'd say she was spot on.
I wouldn't read too much into it anyway. IMO there are so many things that would alter your fundal height, if you need the loo, if you've recently eaten, are you bloated, did the measure from the exact same spot (the amount of times my midwife was going "do you think this is your pubic bone, or here, or here..?")
As long as your scans are good, relax. And if you're under a consultant you'll be taken good care of!