Your lead score is calculated based on basic criteria (intelligent scoring) like phone calls, emails, etc in addition to the custom rules that you add based on your business needs. Examples include, but are not limited to,

• Sending out an email or making a phone call
• Editing the lead's information
• Customer replying to your emails
• Scheduling an appointment with the lead.

The score that you see for a particular lead in the system is the relative score of that lead with respect to all other leads or contacts in the system. i.e, If you see a score of 40 for "John", it means that there are 40% of leads and contacts in your system having scored less than "John". 

So when calculating the lead score we calculate it based on the percentile of the lead instead of percentage. This way the lead score will be compared with other leads and display a scoring which helps you get an idea of the score.