But it is the dilution on a single card, not the entire set. The packs are hopefully the majority of the value held, so the dilution is on that card.

If the packs are worth 10,000 and the first drop is worth 1000, the packs are still worth 10,000, but the 1000 = 900. So, in reality, the 10% loss on total holdings is closer to 1%.