Some enchanted items acquire only one enchantment per time on the enchantment table

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