Is this due to selection bias where only the top paid people would volunteer their salary information or is the data collection flawed?
To be completely honest i don't have a solution for that right now, I just build the site to replace all the spreadsheets that are around there, where people share their salaries in companies to give an idea to other (and themselves) if they are underpaid. I can say that they work realy well by my experience, even though they have the same issue. You need to see the overall picture, and try to get an idea, but don't take the highest and lowest values.
Anyways, I open to ideas, and suggestions of how can I improve the data submission.