EDIT: I was wrong, he was quite down the list! He only appears in the chart because he ultimately won, so higher contenders dropped off.

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He seemed to hover around 1%, which was the second highest behind Tagle (~20%)

https://polymarket.com/event/who-will-be-the-next-pope?tid=1...

That link isn't showing most of the options. I believe there were at least 10 above him. Just individually look at the lines for Zuppi, Pizzaballa, Sarah, etc.

I don't understand what people were basing that on; the conclave is a completely secret process?

The winning lottery numbers are a secret too before they're drawn; people just like to gamble.