Mining, Woodcutting and Herbalism only increase your chance of double/triple drops as they level up. Excavation is the only one in which different drops become available as it levels up, for example: Gunpowder from Gravel at 75, Bones from Gravel at 175, Apples, Eggs and Gold Music discs from Dirt at 250, etc. Check out the McMMO wiki on google for a full list of all skills.