SharePoint Support End Date was always dependent on Service Pack release date and this is how it was before SharePoint 2016. As you know, Microsoft decided to get rid of Service Pack concept and that is why they are introducing new schema.
Starting April 2017 every update will be supported 1 year starting its release date. Microsoft will update the minimum supported build of SharePoint Server 2016 on each anniversary of General Availability (GA)
You can find all dates in a table below