Are the items the postage was applied to in different categories, by any chance?

With a store, the FVF is category-specific. For a basic store, media items (books, CDs, DVDs etc), fashion (clothing, jewellery, even craft supplies) and collectables attract a 9.5% FVF. Most other categories are 8%, then there's tech accessories sold for $200+ at 6% (the FVF percentage drops by 1% as you go up store levels).

So, if an item sold is a book, the FVF on postage will be 9.5%, while if it's a kitchen knife, the FVF will be 8% (or whatever percentage according to your store subscription).