No reason for that impression. The install has been working that way for as long as I've known TC which is I think version 8. Upgrade can fail for many user-related reasons, lack of disk space, etc. I myself am happy with leaving the old version there to start. If TC has deprecated a feature that I used to use, I can still run the old tests until I convert them to something else. We've also had the case where not everyone in the office converted at once and this lets us all keep using our tests.
If you don't want the old version there, then uninstall it. No big deal.