Part of the Codecov static analysis is to retrieve the diff when comparing commits, and use the diff to offset coverage. Coverage may decrease by removing previous lines, which is not uncommon.

Let's show you how this occurs:

hit
  hit
- hit
  miss

Using the diff above, we made a commit that removed one line that was previously hit. The coverage before the commit was 75% (3/4), while the coverage after this commit is 66% (2/3).

Codecov will set the Project Status and Patch Status to success ✅ when it detects that the coverage was decreased due to the offset of the removed lines. Clearly, the coverage is lower, but not due to a developer fault.