That's the upvote ratio. You can test it with a fresh post with no votes, downvote & refresh to see the change. And then upvote and repeat.

It shows how cool you are in here /s

Upvotes ratio. Let score be S, upvotes U, downvotes D and ratio R. Then:

R=U/(U+D) and S=U-D

D=U/R-U and D=U-S

U/R-U=U-S

2U-U/R=S

U(2-1/R)=S

U(2R-1)/R=S

U=SR/(2R-1)

Your current, as of typing this comment, 83% with score of 4 means:

U=83%*4/(2*83%-1)=332%/(166%-100%)=332/66≈5

You got 5 upvotes and 1 downvote