Mysterious Caps Lock key

I found this Caps lock key to be different from every other key in my QWERTY keyboard. Only 4/5th of the key is raised to other keys level. The remaining 1/5th of the key is not accessible to fingers. I searched in google to find out why. I found lots of people who have hatred towards that key and wants that key to be removed from the keyboard. But I could not find any info why exactly the 1/5th of the key is lowered from mean key level. Anybody know the reason?