Erm, can I get a little personal?
I run a pedometer loan scheme here in the UK. I lend out peds to overweight or sedentary people and help them make lifestyle choices to improve their health, weight and appearance.
Many people have brought the peds back because of the problems you describe. The reason for this is quite simple, but a little embarrassing! The peds have to lie at the hip/waist at a certain angle to be accurate. The rounder you are in the middle the more the ped sits at a tilt and therefore becomes less accurate. I have one client who is very round whose ped actually measures his breathing!!!
What we have done is to combine wearing peds with a diary. This means that even if the ped mis-measures your steps you can keep a track of what you did and track your activity levels. So if your ped measure 15 steps for every 8 you take you simply ignore the actual numbers and just keep a record of them to see if you increase the number you take. If you try to recalibrate it every month you should see a difference once you start to change your body shape a little.
This means that although you cannot use the ped to measure the number of steps you take you can still use it for its main purpose - to measure the increase in steps you take!
I hope this helps, and reassures you that you don't have to spend a fortune on a ped. Of course if you ain't apple shaped then your ped is probably busted! But I have to admit that in our experience if they count steps at all then they work just fine on racing snakes!!!!