Hello, thank you for checking out Astah.
The answer is that some are complied with version 2.5 and some are 2.1..
We are trying to keep it updated to the latest version but haven't done it all at once unfortunately.
We update the OMG's UML specification to the latest starting with models that had major changes from previous version and also we implement the update based on user's requests. So technically some models are complied with version 2.5, and some are still with version 2.1. But we believe that you will not feel any lack when modeling UML with Astah. But in case you do, please feel free to let us know, so that we will update Astah to make it work better for you!