FBA inventory age help

Hi, I have been watching my inventory aged FBA stock and it seems to just be getting older each week despite strong sales - eg stock moves from one bracket of 61-90 days to 91-180 days. I have stock in each age bracket but it seems to always be the 'newest' stock (0-60 days) which sells, not the oldest (91-180 days), so my stock is getting more 'aged' by the week. Can anyone explain that please? I assume I am getting charged more for that because it is older and eventually it might even be sent back to me, but that's not fair - surely the 'oldest' stock should always be used for sales first. I am really confused. Thanks