“Bonus Experience – When you kill an enemy in the world, you might see 5 xp + 2 xp pop up . What this means is you’ll gain bonus experience depending on how long an enemy has been alive in the world, which gives you even more incentive to explore.”
Enemies that have been alive (bear with me, that’s a huge article. Somewhere halfway through) for a longer amount of time than other mobs give bonus experience based on the amount of time they’ve been around. This is to discourage grinding in one place.
Note: The amount of XP you receive is also respective of your level.
Source : Link , Question Author : FAE , Answer Author : Jaimin Nimavat