Do the bottles have different colors? Or logos? We have two different systems in Germany: One, where you buy the bottle and one, where you pay a deposit (that are those with a company logo). The deposit bottles you often can only exchange at stations belonging to this chain, the other bottles you can exchange at a lot of places.
 | Difference: If you don't need them anymore, you can give the deposit bottle back to get the deposit. No giving back of the other bottles. |
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I'm fairly sure it's the same system all around here!
But that would explain the difference!