How can a tyre inflate itself?

I recently bought a used car. When I checked the tyre pressure, one of the tyres was over inflated by 16psi. I let the air out to the correct pressure, then checked them all again a week later. The same tyre had increased in pressure by 6psi. How can it possilby be inflating itself? Any possible explanation would be greatly appreciated. I've never come across anything like it before.