What gives... Why can I do chin ups, but not pull ups?

Anonymous
Chin ups are so much easier for me to do, but I can't pull myself up when doing a pull up. I'm having a very hard time with developing the muscles in my back... I'm not sure how to go about this.

chin ups↓
easybuypal.com/.../chin-up-vs-pull-up.jpg
Pull ups↑
What gives... Why can I do chin ups, but not pull ups?
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