Sharpness Enchantments do no effect to a regular iron sword

When I tried adding sharpness 3 or 5 to a regular iron sword with no enchantments...Iron swords do 6 damage and when i finished enchanting the damage stayed at 6 and the sharpness enchantment was listed in the sword!
[IMPORTANT] But this happened to me in 1.7.2 but it didn't exist in the affected versions box so i put in another version.