This software update does not really exist:
The updater often knows about new updates before using 'Check for updates' in the software itself does.
If .64 wasn't pushed through the built-in updater it may not be pushed at all, .66 was built hours later.
I'd rely on the browser updater instead.
Browser updater shows:
Software releases are often staggered, helps mitigate risks for example, but there are more reasons.
Since .64 hasn't been seeded through the updater (for you, it could have for other users) and .66 was built less than a day after .64, it is likely that .64 won't be made generally available; they might have caught a bug in .64 that prompted a new build, and even for .66 perhaps they're just waiting a bit before flipping the switch so to speak.
They are all available at Opera's FTP servers if you want to get them already, I generally wait for most programs that have auto-update mechanisms to do their thing.
OK, I will wait.