I've recently been enchanting my armor and I noticed that when I enchant an item it usually comes with another enchantment. For example, I click on projectile protection II but when I enchant it I also get Unbreaking III. I am playing in Minecraft 1.12.2 with forge and three mods. However, these mods are not supposed to affect enchanting. They are ruins mod (generates structures), nomadic tents (adds tents), and miner's advantage (makes mining quicker). Is this normal?
The Preview is only that, a preview
This is perfectly normal.
The Wiki has a lot of information on probabilities and algorithms.
Long story short: When you place an item on the enchantment table it shows you one of the enchantments. It could be one out of one, or it could be one out of four, or in between.