Yes, because you are a rock climber. Every climber I know is like that. They have ridiculous grip strength, they are great at pull ups. Bodyweight exercises are easier for lighter individuals. Just look at people like Frank Medrano, I'm sure you've heard of him, he's a vegan. He's strong, but compare him to a Chinese Olympic lifter of the same weight...lol. But even the climbers I know are much heavier than you, they sit at around 65 - 70kg. You are very underweight my friend.
I know guys that can deadlift 300kg, bench 180kg, squat 270kg, and they would struggle with 10 pull ups added with 20kg. Because they have bodyweights of 100kg+. It would be silly to call them weak.
I'm not saying you are weak, but other than pull ups, how strong are you on other exercises? Such as, for example, squat, deadlift, bench. I'm guessing you don't do them, huh?