Garcia is a very serious puncher. I've known about him since the amateurs even than he was known for his strength. There's no shame in being rocked by him. You can block his shots and they'll hurt.
Khan has special talent and ability. Some of the fastest hands ive ever seen. If it's that fast on tv, his opponent isn't seeing any of those punches. But as long as it's predictable where his head is he will always be a candidate to get ko'ed by guys who can crack.