As I'm preparing a new FREE DLC, I decided to make all the original levels available for all new players. That means, you don't have to play the game in the original order of the levels, you can choose what levels to play!
This resulted in grouping all the original levels together into Season 1, and creating individual achievements for each level.
As the game gets updated with new content, I need a better way of organizing it all. Also, I think it's fair for everyone to enjoy the full content that they paid for, regardless of skill.
The new achievements system is based on the completion on each individual level, rather than the linear progression it had initially.