why GET \commits?since=<commitId> gives commits older than the sinceCommitId param value?

[nil2mahadik]

I am querying a REST API /commits?since=<commitId> with an expectation to get all commits after the mentioned commitId. 

This rest client works as expected for feature branches of project however for master branch where we get PullRequests merged from peer developers, this call gives some older commits also in result of this call.

My question is why so? and what could be the root cause? what are the alternatives available for me?