Really, though, I don't get it. Why use not uncommon when one could just simply use the word common? I mean, they practically mean the same thing--to me, anyway. Is there some degree of commonality between common and uncommon? To me, it just seems that something is either common or it's not, in which case it'd be uncommon.

For example, "It's not uncommon of find a three-leaf clover." (x3)
In my mind, that's not any different from, "It's common to find a three-leaf clover."
Common is the opposite of uncommon, so not uncommon would basically be common, wouldn't it?

Also, not uncommon would be like a double negative, right? But I know there are ways to use them effectively to emphasize something, which I suppose is the point in this case.

Again, I don't know if there are certain degrees of commonality, since there are those likely, not likely, impossible, certain things, but I was always curious about this. Anyone with good knowledge on this? x3