We have been here before.
In all the censuses there was a separate series of enumeration books for the various Islands around the UK mainland. These books were of slightly different format to the normal books.
The first Island in this series happens to be Anglesy so if the transcribers are not careful or knowledgable subsequent Islands can become listed under Anglesy.
Other problems exist when parishes are listed under odd names. Always look for Conchan and Onchan.
Even though the four towns on the Island were enumerated separately, they are often listed under the Parish. Douglas for example can appear under Douglas, Braddan, Onchan or Conchan.