gitlab branch compare is different fom git diff

Summary

gitlabe compare does not show the actual diff between two branches. Cherry picking a change from one branch to another does not exclude it from the diff.

Steps to reproduce

create a feature branch. Apply a change to this branch. (change A)

apply another disjunct change to the original branch. (change B)

cherry-pick change B into the feature branch.

compare feature branch and original branch in gitlab. The comparison will include change B even that it is part of both branches. It shows the original branch in a state as it would not include change B.

Expected behavior

I expect that compare shows me the actual difference between two branches. The same way as git diff does. Here: git diff between feature and original branch only show change A.

Actual behavior

The comparison will include change B even that it is part of both branches. It shows the original branch in a state as it would not include change B.

Relevant logs and/or screenshots

lefttop gitlab comparison. Right file at feature branch and master branch. Left bottom git diff between feature and master branch.

GitLab Community Edition 8.14.5