It's fairly common in areas where there are number of private schools, where schools can usually predict as they say - a number of pupils will drop out to go to private in the end. Several schools around me regularly do it. Mostly it works, sometimes it doesn't!
The issues for them at appeal is that appellants can indeed say, well, you offered 251, so you have to be prepared to deal with that - which is true, to an extent, but it doesn't mean there wouldn't be prejudice to the education of other pupils to do so.
A PAN of 232 suggests 8 forms of 29. 251 would suggest something like 9 forms of 28.
The interesting point comes where acceptances drop back to, say, 236, meaning 4 forms now have 30 - so an appellant could say, well, what would be the prejudice of adding 1 more to another form as well? There would be some, but the argument could well be made that it was low.
A question to ask on the day would be what are their current acceptances to date.