Issues with SORT

Hi all, its been a while since I popped in here last, how are we all doing ...

OK moving on - I have an issue with SORT, its kind of difficult to explain, but hear goes - sometimes when I click inside the selection for my sort, then click sort, select custom, click my sort has headers - under sort by etc - it shows (column C) ...
the attachment shows what I mean. Unfortunately I cannot copy up the spreadsheet due to data protection. I am using Office 2013.
Any ideas, it does this randomly.