I get inconsistent, confusing results for evaluation score.

I did the levequest "Catch My Drift" in Coerthas several times on max difficulty when I was level 35-39. At this point I was getting 7 or 9 evaluation score for every gather, (depending on whether I went for the low or high chance resource). This was enough to quite comfortably get +25% EXP. The reward for the quest was huge, roughly 35,000EXP (20,000 base, +15,000 for difficulty/speed/+25%).

However, I came back to this levequest later, at level 42. I did the quest on the same difficulty as before, thinking I'd be able to coast through it with my better stats and gear. This time, however, I only gained 4 score for the easy resource and 6 score for the more difficult one. This made it very difficult to get a high score and I wound up hitting +10%.

The same thing happened for"A Stash of Herbs" too

Can anyone tell me why my evaluation scores are dropping? It seems like it should get easier as you level up, not harder