It's not worth twice as much. There was a buyer willing to pay twice as much. For something like this, the fact that value is completely subjective really sticks out. Trying to sell it again tomorrow might go for half or double.

To be fair that's true for everything including the jewels

If the buyer paid 9M, doesnt this means that someone else wanted to pay 8M?

What I am trying yo say: at least two collectors (?) or speculators (?) with deep pockets.

Yeah, the point of sharing was to point out the subjectivity. The brooch sold at auction too.