Hello,
Whenever we submit a change in Gerrit, the patchset parent is most probably not the latest version of master. Hence Gerrit happily merge our patchsets and thus generate a merge commit. That sometime confuses people. Note that one can just 'git log --no-merges'.
Gerrit has an option to enforce "Fast Forward Only", that would require us to press the [Rebase] button before submitting and will result in a history which is less confused to those not filtering merges commits. Drawback is that when someone sends a serie of patches, they will need to be fast forwarded and thus we will somehow loose the fact that those commits are closely related.
My opinion is to keep the current setting and manually rebase before submitting. That let us the opportunity to have Gerrit craft a merge commit when it is actually needed (aka merging a serie of related patches).