I noticed two thing swith the Gruok west of Kion.
1) XP from kill, I'm a level 8 warrior and I'm getting "This enemy is not worth any experience for you"
2) Ishalin quest for "pesky gruok", the count is not changing when killing gruok.
add note ... the sand beetle & water golem's SE of gruok (near brown spider island) are also "not worth xp to you". Might be an area thing.