The overall rating is calculated based on the all ratings published recent one year.
This means that if a profile was rated negatively 2 years ago, that rating will not count toward the overall rating. In other words, a profile can be rated negatively 2 years ago and positive 6 months ago, that profile will have an overall rating of 100% because it is only calculated for the last year.
The idea is to show the rating in the trend of the last year because any profile can be better than a year ago and vice versa.
Read How to write review and rate the profile for more information.