If it's in the UK, the it's probably all down to the fact that there is no standard position for a 'centre fix' in the UK. Some use 14.5 miles, some use 12, some use 10, some use less. As a result, we are NOT allowed to clear a pilot to his/hers centre fix, simply because we have no idea where it is.
To get round that, when we are willing and able (after co-ordination), we can send pilots to a generic centre fix, which is nominally 10NM. If you need more, or less, then say so when we clear you. It's not a big deal to position other traffic as a result if you need more or less miles on final. We do need to know though.