Multiple items in same year by same author not listed correctly

On the laptop I was testing it on, I was comparing the installed Cell - got (Author, 1998a, b, c) and the installed Harvard --got (Author, 1998a, Author, 1998b, Author 1998c) which gave the correct a b and c-- while APA 6th gave (Author, 1998a, Author, 1998c, Author 1998e)  – and I tried changing every APA 6th setting I could think of - and it didn’t appear to affect the assignment.