Why do normal mobs....
It may have been that the reef rider you were fighting the first time belonged to an event, and got upscaled becouse of the number of players partaking in the event.
While the second, veteran was a normal one.
Just a guess.
I got the same experience as Namu, the normal one even with the two of us took longer to kill than the veteran. And they were not event ones, they seem to be normal ones roaming around pearl way outside the resort.