Naylia hit the nail on the head.
I understand why they put the 'claim only one monster/group' into effect because otherwise people can hog monsters left and right. However, if you accidentally aggro another monster, or group of monsters in a guild leve, you have to make sure to kill the first group while taking a beating and not touching the second group, or you get less SP for doing so on the unclaimed monster.
Or another examples which annoys me to death, is during a pursue leve. The monster runs away after you've been attacking it, becomes disengaged and doesn't regen to full HP, therefore, you lose SP. It's sooooooo annoying, and I personally think it's ridiculous to have to wait for the monster to regen to full health before getting full SP. I can't tell you how many times I've died with the monster at 1%. So what are you telling me that all that previous work was for nothing? Bogus.
Pretty much agree with exactly what Naylia said, I just wanted to elaborate on why it is annoying.