I found too much fossil ore in my world for my liking, so according to the tooltip I increased the rarity to 100.
A while later it still felt like too much (in new chunks), so I increased rarity to 300.
A while later it still felt like too much (in new chunks), so I increased rarity to 900.
A while later it still felt like too much (in new chunks), so I took bdlib and measured that the fossil ore percentage, by going from 900 to 10000, increased from 67% to 97%... thus by setting it to 10 I finally got what I had wanted in the beginning. Peace. A better life.

Yes I am writing this into the issue because I want you to know this has been bothering me for about 8 months.

Higher number does not equal more rare. It does not.