The way Realm encryption works is dependent on the size of a memory page - usually that's different between 32bit and 64bit devices (or more specifically the architecture of the app, not the device).

We should switch to a fixed size as part of core 6.
Historically the size had to be a multiple of the mprotect granularity (which isn't actually the same thing as the page size on ios) and we didn't want to change it for 32-bit for compatibility reasons, but that's no longer applicable.
The hmac makes it so that we can detect the page size used: just try to decrypt with both 4k and 16k page sizes, and whichever one matches the signature is correct.

The issue is experienced in e.g. Studio: https://github.com/realm/realm-dotnet/issues/1779#issuecomment-431079582