I've wanted to understand what on earth this number is supposed to mean as well. I wholeheartedly agree with Streetwind that the game should stop displaying the number entirely because it taunts the player to understand it while being impenetrable.
I used Claude Code to analyze the mod code Streetwind linked.
I know use of LLMs is highly controversial. I understand if people need to register their disagreement/disapproval, but in general I propose skipping the post and moving on if you don't care about its analysis. I'm putting it here "for what it's worth".
According to Claude, a 2.5% would mean that you'd expect to find 25 blocks out of every 1000. It confirmed what we all knew that it is not measuring the actual ore present, just generation metadata.
Maelstrom's point about "Other conditions like caves will impact that number" is well taken; you'd hypothetically find 25 blocks on average assuming the entire column was eligible, but things like caves make some blocks ineligible, while probably not being factored into the number.
Transcript attached.
claude-code-vintage-story-ore-density.txt