Why can I not choose Expert or Evil for some types?

The studio only offers levels it can honestly reach and a printed book can explain:

• Classic 9 x 9, Sudoku X, jigsaw, samurai, thermo, arrow and 16 x 16: Easy to Evil.
• Killer: Easy to Hard. Harder killers need techniques far beyond what a book can teach, so the studio does not promise them.
• 6 x 6: Easy and Medium. Harder 6 x 6 puzzles hardly exist at this size.
• 4 x 4: Easy only. Every 4 x 4 is solved with the simplest step.
• Kids: 4 x 4 Easy, then 6 x 6 Easy, then 6 x 6 Medium. The card shows a short note for each type.