I'm trying to understand if player races and monster races with the same abilities function differently. I had this issue arise with player regarding a monster race and a player race not having the same ability.
In the MM pg. 122, the Duergar monster race has this ability:
Enlarge (Recharges after a Short or Long Rest). For 1 minute, the duergar magically increases in size, along with anything it is wearing or carrying. While enlarged, the duergar is Large, doubles its damage dice on Strength-based weapon attacks (included in the attacks), and makes Strength checks and Strength saving throws with advantage. If the duergar lacks the room to become Large, it attains the maximum size possible in the space available.
While in the SCAG, the Duergar player subrace have this ability:
Duergar Magic. When you reach 3rd level, you can cast the enlarge/reduce spell on yourself once with this trait, using only the spell's enlarge option. When you reach 5th level, you can cast the invisibility spell on yourself once with this trait. You don't need material components for either spell, and you can't cast them while you're in direct sunlight, although sunlight has no effect on them once cast. You regain the ability to cast these spells with this trait when you finish a long rest. Intelligence is your spellcasting ability for these spells.
The issue that arose is that a player who is playing a duergar and used enlarge on himself had to keep rolling for concentration when attacked, but the duergar monster that was also enlarged whom he was fighting did not.
My question is why does a monster race and a player race have two different abilities? Shouldn't they be the same?