Anything that measures is subject to a built in measurement error.

The speedometer on your car for example might say 40mph but on reality the true speed is somewhere between 39 to 41 mph

Also all measurement devices wear out. In the case of energy meters this may result in higher or lower readings. If your old meter has been "reading high" then you've been paying for energy you haven't actually used. The reverse of course is also true, your old meter might be under reporting so you've had a bit of free energy.

The bias on the meter can of course be checked and adjustments made, that's what calibration really means but it's an expensive thing to do. It's much easier to just replace the meter for a brand new one ie with a state of the art smart meter.

Under current arrangements you can still refuse a smart meter but your energy supplier will still want to replace the old meter and put in a "dumbed down" meter with less capabilities.