Beyond superlatives

My son and I went skiing the other day and he asked me why double black diamond trails which are billed as "Extremely difficult" are more difficult than black diamond trails which are described as "Most difficult". He said, "How could there be something more difficult than 'most difficult'?"   Is "most difficult" more difficult or less difficult than "extremely difficult"? According to the New Oxford American Dictionary, "most" is defined as "greatest in amount or degree", whereas "extreme" is defined as "reaching a high or the highest degree".  So there is a sense where "extremely difficult" is the same as "most difficult", but there is an interpretation of the definitions where "extremely difficult" is less difficult than "most difficult".  Perhaps the wording in the ski resorts should be changed to more clearly indicate which trails are more difficult.