I may be a little off here, but as far as I can tell, the mini USB will only charge through the computer and the only one that will charge through the AC plug is the one that's round.
This can then be plugged into a USB wall charger to charge the reader.
If the mini USB is connected to the book for any reason ( to conect to the computer or just because you attached a mini USB by mistake for no reason, even if it's not connected to anything on the other end) it will go through the whole process of loading. Sometimes taking a long time, depending on how much you have in your reader. It onl charges through the computer and NOT through a wall charger.
If the round charger tip is used, it charges, and that's it. And it doesn't go through the loading process. Seems much easier to me.