In the Buildbot world, it means that an infrastructure issue happened and the job needs to be tried again. In TC, both an infra failure and a test failure end up turning blue. This makes it harder to spot intermittent failures, hence, being reported and keeping track of them to get developers to investigate it. What is it required to change the current blue colour to orange? See https://treeherder.mozilla.org/#/jobs?repo=gaia&revision=09d58fa42fd0db566ee322f29a637153532c1421 for an example.
So I believe this is because "retry" and "rerun" for a task both result in a job status of "retry" being submitted to treeherder. At least that's my quick glance at it. https://github.com/taskcluster/mozilla-taskcluster/blob/6a3676c20ec988414a46986de7ec45fb4f552e2a/src/treeherder/job_handler.js#L341