The progress bars are based on sales made. The more times a certain game has been bought, the more you have to play it to unlock it.

For example, 1 2 3 KICK IT! was released at (nearly) the same time as the pack, so it has the fewest sales making it easier to complete.

Killing Floor and Audiosurf were out for longer and as such have had more sales, so they require more hours to complete.

According to Dougley, Potatos still matter. Get them.

The Store should keep a record of how many times a certain game has been bought, so we need to go find that and order which games we should play based off that.