Because the list isn't maintained well.

For the same reason we have books with a dozen leveled pieces, and only ONE of those pieces shows up on the list, even though five or six of the others are of equal difficulty and should be on the same list.

The S&E PML needs to be wholly re-examined from the bottom up, starting with the Grade 3 lists. Hire a team of private teachers and college professors to revamp each instrument.