I wondered if the discrepancy was usual or a problem.
Both! (Possibly.). It’s most likely that the vendors or some other previous owner(s) have carried out works (extensions, extra floor, roof terrace etc) “illegally” and so the additional sq m has never been recorded or registered.
This topic has been covered a few times before if you carry out a “search”.
You should ensure that the vendors rectify the situation before you commit to buy and that they pay any outstanding fines, back taxes etc otherwise they come with the house.