My Fitbit records way more steps than I actually do usually. To test it I left it off until I was about to drive to spawn's then I put it on in the car with 0 steps. By the time I arrived at my mum's to pick her up it had recorded 80 steps and I sussed that it gives me steps every time I turn the steering wheel. When I got to spawn's I'd done over 800 steps despite not leaving the driving seat of the car.
I think the problem with the Tesco steps might be that I took Mr F with me so it was very, very slow, more than twice as long than if it was just me, with lots of stopping while he rested. I tried to get him to take the crutch but he refuses to use it until he absolutely has to, I even pointed out a younger man than him who was using a walking stick but my argument failed because "yeah, but he's wearing fecking crocs! Freak.".
I've now done 1878 steps today.