i have been trying to enchant my gear for some time now, and i have noticed that in the enchant table some groups of items get the same enchants on the table GUI. 

these are the groups i have noticed:

chestplate/leggings

helmet/boots

sword/axe

pickaxe/shovel/hoe

also, everytime i try and enchant the chestplate with thorns 2, i always get projectile protection 4 along with it. it has been the same process for 4 times now, and im not sure why this happens.