Ah but that's to do with heat, I'm pretty sure, making it a bit loose until it cools down. You shouldn't be moving your preserves until the jar is cool anyway. Can be dangerous if you put a hot jar on a cold (or wet) countertop because the extreme temperature change can cause the jars to crack.
Going back on topic... the bottles snap down dangerously strong compared to the jars. You don't want to get your fingers pinched in those clasps. And I like the ikea preserving jars/bottles because they have silicon seals, not rubber, so are less likely to wear out too.