Why should ether O be named as OXA in a bicyclo ring when it is a substituent? I was asked to name the molecule below .

For the following molecule, since ether is a substituent here (keto is more prior group) my answer was:

3-methoxy-6-methy-7-oxy bicyclo[2.2.1]hept-5-en-2-one

.

My answer is wrong. The correct answer is

3-methoxy-6-methyl-7-oxabicyclo[2.2.1]hept-5-en-2-one

My point is, at C-3, ether is methoxy, but at 7 the same ether is OXA. Ether as substituent outside the bicyclo ring is alkoxy, but inside bicyclo compound it is OXA.
My question is, why should ether O be named as OXA in a bicyclo ring?
The suggested name 3-methoxy-6-methyl-7-oxabicyclo[2.2.1]hept-5-en-2-one is correct. Indeed, the ketone is the principal characteristic group in this case; however, this is not the reason why the other oxygen is named as ‘oxa’. Already the unsubstituted parent structure is named 7-oxabicyclo[2.2.1]hept-2-ene.

According to the current version of Nomenclature of Organic Chemistry – IUPAC Recommendations and Preferred Names 2013 (Blue Book), such bicyclic heterocycles are named using skeletal replacement (‘a’) nomenclature (a system in which a heteroatom replaces skeletal atoms of a parent hydride).


  P-31.1.4.4 Bi- and polycyclic von Baeyer heterocycles named by skeletal replacement (‘a’) nomenclature. In heterocyclic compounds formed using skeletal replacement (‘a’) nomenclature, low locants are assigned to heteroatoms, in accord with the fixed numbering of the system, then to unsaturated sites.


Nondetachable prefixes, called replacement (‘a’) prefixes representing the element being introduced, are used to designate the replacing skeletal atoms with their standard bonding number, for example ‘aza’ (N), ‘oxa’ (O), or ‘thia’ (S).