Cumulative Doesn't Mean Cumulative

So, I just tried adding a new server to a farm, after patching it to the same level as all other servers in the farm (April 2017 CU).

Surprisingly, the SharePoint configuration wizard told me off, saying the server is missing the December 2016 CU.

Yes, that is right - the April 2017 CU does not include the CU that was released 5 months before it...Cumulative? nope!