A perfect notes to understand SSD.
I believe that your planning horizon is ending by Dec 13. As ATP based on planning output, planning is not doing any calculation beyond Dec 13. Another issue might be - the way you are doing over-ride ATP. As you mentioned, system is showing Dec 13 date. When you are trying to ammendemnt to 2014 then it is coming back 2013. This tells me that either OM; Override ATP profile option is not enabled or you are not over-riding this date by clicking 'Over-ride ATP' column on SO line.
Hope, this will be helpful to you.