There are several differences between the Nexus artifact as a normal task and as an artifact task. Some of these differences are:
When you select an artifact task, the Job has a reference to Nexus. For example, you select the latest version of say, 0.3, of the Job in Version while adding a task. Then you change the Job in Studio and publish it with a new version of say, 0.4. The artifact task downloads the latest version, that is 0.4, before running it in the JobServer, since it has a reference to the Nexus. However, a similar Job selected through normal task (Nexus), still uses 0.3, because it doesn't have a reference to Nexus.
The context in the artifact task is taken from what you select while publishing the Job from Studio.
You don't have an option to change the context of an Artifact Task.
You can change the context variable of a Normal Task (Nexus).