The official explanation includes this passage:
After checking to see if a potential Juror is not excluded, the software then does a virtual "dice roll" against the member's chance of serving. Every member's chance of serving is represented as a percentage and is publicly displayed on their Profile page. A member's maximum possible chance of serving is 100%, and their lowest possible chance of serving is 0%.
Things that increase a member's chance of serving:
Increase 1 percentage point for each 100 posts (up to 20 percentage points max = 2,000 posts)
Increase 1 percentage point for each 10 days of membership (up to 20 percentage points max = 200 days)
Increase 1 percentage point for each post in the last 90 days (up to 20 percentage points max = 20 posts)
Increase 40 percentage points for Star Membership
Things that decrease a member's chance of serving:
Decrease 20 percentage points for each hidden post in last 90 days
Therefore, if you aren't a Star member, the chance that, when you're randomly picked, you'll actually be invited to serve, maxes out at 60%.