I find when I'm listing a lot of different items the weight updates do (sometimes) revert themselves back to the previous listings weights when you're not watching. It's just another ebay glitch. To solve it I keep a couple different weight items I've already listed in separate tabs on top, and just "sell similar" whichever is in the correct weight group.
It can really mess things up if you don't catch the errors, and the customer pays for a much lighter parcel!