I only use eBay auctions and as I rarely use reserves I have to add that this behaviour goes back a few releases (can't speak for release 70 but if it hasn't looked into until know the problem should still exist).
When you list an item with a reserve on eBay you pay an extra buck or so for the reserve. You get it back when your item meets the reserve. Aw2000 correctly counts this extra fee at the start of the auction but it doesn't recalculate the final fee correctly when ie. the reserve is payed back by eBay because the item did meet the reserve.

Howard
Wheel of Time