What is the difference between over function "Next" and "NextPeriod"?

What is the difference between over function "Next" and "NextPeriod"?

The over functions "NEXT" and "NEXTPERIOD" both use the next node on the same level as the current node.If there is no next node, that is, if the current node is the last node for the current level, the resulting subset will not contain any rows.

So what is the difference between these two functions? If you have a time hierarchy and you don't have data for one "Month" you will notice a difference between Next and NextPeriod. 

For example if you have "Jan", "Feb" and "Apr" data for 2015 and "Jan", "Feb", "Mar" and "Apr" for 2016 refer to the screen shot below.


Dxp file attached for reference.