Why do they make serial numbers case sensitive when there are only upper case letters?

You know when you install a program you will in many cases have to enter a serial key. And some of those will be case sensitive. Now, i just installed Caesar 4, one of the examples of case sensitive serials. A serial without any lower key characters. Why do they make a serial case sensitive if there are only uppercase letters and not a single lower case letter? I doesn’t make any sense to me.