It is easy to see how the confusion arises.
The IoM census returns appear in a series of volumes entitled "Island off the British Coast" or something similar ( I can't remember the exact wording.)

It so happens that the first Island in the Volume containing the IoM is Anglesy.

If you don't know what you are doing, it is easy ( though hardly forgivable in an allegedly commercial genealogical site) to use the name of the first Island encountered for all.

I know the first time I ordered a film my reaction was "they've sent the wrong one". Luckily I kept looking.