Usually and Unusually are not perfect opposites, despite appearing so based on their root and prefix. Indeed, usual and unusual are mostly the same, but somehow making this adjective into an adverb changed that.
Just think about it:
- Usually means "on a regular basis", "often" or "most of the time"
- Unusually means "strangely enough" or "abnormally"
Then there's flammable and inflammable, which have the same meaning - but that's another story.