How to calculate NFT rarity score?

I want the correct calculation for how to calculate NFT rarity score? I checked few sites and articles. As per their rule, Calulating a score of the NFT is "Sum of the Rarity Value". But when i checked different sites, like opensea, rarity.tools, rarity sniper; all has different score value of same NFT. Like, Bored Ape Yacht Club has rarity value as (13,2,1,1,2,2,0.49) for NFT ID 7495. As per rarity tool, rarity score is 333.86, NFT sniper has 4674.07. I don't think this calculation is meaningful.. Or the meaning of rarity is something if there is different value on different sites for same NFT. If there is actually a meaning of calulation of NFT rarity..it must be same on every websites for same NFT.