The way that the stars and flames work is a little more complicated than that because they take how relative the score is in comparision to other leads in the system.
The stars represent relative score -- a measure of how this person's lead score compares to everyone else's. A high relative score means this person is probably more interested and informed about your offer compared to people with lower relative scores.
For example if you had four leads
A - score 0 - no stars
B - score 15 - 1 star
C- score 50 - 2 stars
D - score 80 - 3 stars
So you can't say for sure that eveyone above 75 will have 3 stars because if 80% of your leads have 76 points and a handful have a score of say 90+ then those handful, relative to all the other leads, would be your best bets and therefore should have more stars than those with less point.
I hope that helps.
http://community.marketo.com/MarketoArticle?id=kA050000000KyvXCAS