Mistwell
Crusty Old Meatwad
From the book:
"A passive check is a special kind of abilily check that doesn't involve any die rolls. Such a check can represent the average result for a task done repeatedly..."
Average is not minimum.
And "can" is not "must". There are other more specific rules regarding passive perception specifically which overrule the general example rule of passive checks (which is not an exhaustive list of how passive checks work, nor is it phrased as if it were).