Continuous Enchanting

When I put a pickaxe in the enchantment table and hover over level 30, it showed Silk Touch I, so I naturally took it. However, when I put the pickaxe back it, it showed Silk Touch I in 30 again. I thought it was strange, but the next two gave me silk touch, also. I thought I was lucky, but then I realized it did the same thing for the other two enchantment tabs and worked for swords (and probably everything else), too. I did this in survival and did not try in creative.