Recently, when my cat was sitting on my sister's lap, my cat leaped off onto the floor and accidentally scratched my sister under her pants. It was about an hour later that my sister noticed the scratch had been bleeding slightly, but she didn't find any damage to her pants. She's now wondering how cat claws can break a person's skin and make them bleed without causing any visible damage to the clothing covering the scratched area. Given that this is such a specific question, she can't find any information about it anywhere.
I'm guessing the pants are made of a woven fabric. Sharp pointy things like claws can get in between the woven threads and go straight through, and no damage is done to the fabric. It's why when you sew a woven fabric, you pin the parts together before you sew them, and no damage is done to the fabric. You don't pin fabrics that aren't a weave, like leather, because you'll make permanent pin holes.
Since the cat was jumping off her lap, the cat was probably instinctively digging its hind claws in to get purchase. This would make the claws more likely to get through the weave of the fabric than with other motions, like swatting with a paw.
I recommend regularly clipping the cat's claws, because the duller they are, the less likely they'll scratch through your clothes.