Canada as a Commonwealth member still uses the term "crown" to mean basically "the government" in most all aspects. What the Canadian government claims as copyright is still called Crown Copyright in our lawbooks and it will probably remain so as long as the British soverign is also our ceremonial head of state.
It would apply to works created by government, created by others by commission of government, or works that were ceded to government or the people by the author or owner. My guess is that the publishing rights to the Leach collection was either commissioned or ceded, probably the latter. Most of the material in the collection would be PD in any case.
There has been much argument here in the past about the governments right to restrict access to public owned documents. Classified documents are already well protected by other legislation so this copyright is only about revenue. As best that I can understand the application is that non commercial duplication is permitted but if the government smells money to be made they reserve the right to collect.