In "HKEY_CURRENT_USER\Software\Opera Software" for the user that ran the installer last, you'll find "Last Stable Install Path", "Last developer Install Path" and "Last beta Install Path". When you launch the beta installer, it defaults to the path the last beta install path was (if present). If you previously installed the 32-bit version (on accident in your case) to "C:\program files (x86)\Opera beta", that's what the beta installer will default to, even if it's the 64-bit installer.
If upgrading a 32-bit version with 64-bit and you don't explicitly change the install path, it'll upgrade the 32-bit in place instead of deleting the 32-bit files and installing the 64-bit files to "C:\Program Files\Opera beta". Although this is annoying for some of us, it'll work just fine.
To avoid all that, always explicitly download the 64-bit installers on the right side of https://blogs.opera.com/desktop/ and always click "options" in the installer to set the "install path" to the correct path.
You can also download from:
ftp://ftp.opera.com/pub/opera/desktop/
ftp://ftp.opera.com/pub/opera-beta/
ftp://ftp.opera.com/pub/opera-developer/
and pick the x64 setup file.