Some items like diamond pickaxe and diamond chestplace are get just one enchantment per time on the enchantment table. I put them on a level 30 option and they just get one enchantment while other items as a diamond sword and a diamond boots get 3 or more as it usually happen at this level. I tried to enchant a diamond pickaxe for 3 times and in all it acquire only one enchantment for example. I don't tested others items yet but i guess it happens with others too