Yes I expect the accuracy may not be down to the nearest 1p and if the true value is intermediate you will also get rounding errors. But evidently you are in the UK so 240V applies. Or 230V - seems there was some fudging done to achieve EU harmonisation some years ago, whereby the official nominal voltage in the UK dropped, at least on paper, from 240 to 230. I think the real voltage varies a bit: it's the frequency that has to be rock-solid, so voltage can be allowed to vary with load to some extent. But in the UK your kettle won't be 250W, for sure. It will be 1kW +/- a few %.