Cay says on the website that sharding is working and the team has access to a new world, on a different server, for testing. Given that, it is entirely possible that Igel was on this world, on its own server, and was experiencing 20 TPS, while everyone else, on S3 was experiencing something considerably less. If you do /list medieval it shows how many people are on the different worlds. When sharding is implemented each world will have it's own server, so the TPS will be based on the present results of /list medieval instead of the total # of people on the server. They will then increase the player cap to balance max # of players and TPS (probably based on the most popular world, though they may have other ideas).