I’m curious about this as well. I mentioned earlier in the thread that I paid extra for a sealed set. This is the one I bought (the text, 未開封, says “unopened”):
One thing which is interesting to note is that the packs have holes near the top which appear to be for hanging these cards up individually. All of the sealed listings I’ve seen have the exact same packs - so I think we can happily rule out third-party sealing.
I had originally assumed that these are available individually from the Munch Exhibition and the unsealed ones are from the Pokémon Center. Why would anyone visiting an art gallery be interested in buying an entire box of Pokémon cards which have nothing to do with the exhibit just to acquire a card which does? However, the listing I bought mine from explicitly mentions that these were a bonus for purchasing the GX Ultra Shiny box, so maybe not.
This sold listing on Rakuten mentions that they were acquired at the Pokémon Center: item.fril.jp/7d32f29ee57a7856e37b19cdc6918889.
This sold listing on Mercari mentions, “after putting it out of the bag I put it in the sleeve”: item.mercari.com/jp/m66362606848.
Perhaps it’s just the sellers themselves opening them up before selling.