I was grinding a few instances on my warrior real fast between some questbot and when I tabbed back I saw this at 15m xp/hr. I took a screenshot after I stopped laughing.
Well, you need to leave it going longer for it to make an estimate that is more or less correct.
as you can see from your screenshot is that it has killed 3 mobs, prob in a very short amount of time boosting the XP/H up with alot.
Or ofc you have started it and nuked down 3 mobs. who knows
Kidding? You are in the Questing mode.. so your Char turned in a quest and got this EXP Boost. You are between Level 60 and 70....You gained something about 4k EXP at this time.. ->
15 000 000 / 60 / 60 = 4 166,66667 ...