Right now each Diff of Differential Revision has associated Harbormaster Build that holds lint/unit test results. Diffs created before this Phabricator code change was introduced are shown as having 100% of code non-covered:

This happens before for case, when Harbormaster Build was not found (see https://secure.phabricator.com/diffusion/P/browse/master/src/applications/differential/controller/DifferentialChangesetViewController.php;7ab970d1a7d347d2d318f9cedbe11ac75addfa3a$403) the coverage is array() (0% coverage) instead of null (no coverage information provided).