There's a story behind that. As I recall, King George Third refused to accept that IV was the correct designation and decreed it would be IIII. The story is also attributed to King Louis XIV of France.
I prefer this reason: using IIII means that only I is seen the first four hour markings on the, V is only seen in the next four markings, and X is seen only in the last four markings, creating radial symmetry.
So it's really an artistic licence.