The Interview Difficulty is a score ranging from very difficult (red) to very easy (green) generated based on the Interview Insights at this company.
The number you see in the middle of the doughnut pie chart is the simple average of these scores. The higher the number, the more difficult the interviews on average. If you hover over the various sections of the doughnut, you will see the % breakdown of each score given.
The percentile score in the title is calculated across the entire Company Database and uses an adjusted score based on Bayesian Estimates (to account for companies that have few interview insights). Simply put, as a company gets more insights, the confidence of a "true score" increases so it is pulled closer to its simple average and away from the average of the entire data set.