Why is in Natty old version of Transmission 2.13?
Current version in 2.22 released on March 4th.
It hasn't made it's way into the Official Natty repos yet, because for the release, they need to have things stable. I believe that Transmission was part of the Debian Import Freeze, however I'm not 100% sure on that. It could have been part of the Feature Freeze back in February, but again, I'm not sure.
Above info is found in the Natty Release Schedule Wiki Page However, if you're feeling adventurous, you can take a look at this question:
The PPA you want to add is this:
from the Transmission site