Exactly.
Try to do barbell curls with your back and elbows against a wall. You'll probably struggle to move 80-85% of your usual work set poundages.
That's essentially what you're doing in an incline curl: with your elbows so far behind you, it's much harder to curl the same weights (if you can at all...I can barely incline curl 60 pounders, but I can do a few strict reps with 70's in a regular standing DB curl).