Think of ‘Magazine issues’ as something that is created within each ‘Magazine'.
This is to accommodate multiple ‘Magazines’.
For instance, if you have a ‘Monthly Magazine’ and an ‘Annual Magazine’ we can create two ‘Magazines’ (say, Monthly Magazine, Annual Magazine), and then add issues under each separately (e.g: June 2016, Jan 2005 under Monthly, and 2008, 2009 etc. under Annual ).
You can add a new magazine from Magazines on the admin panel, and then click on View Issues to add each issue of that Magazine.