It is called "variegation" and is caused by a slight vascular problem in the affected leaves.
It does not pose a problem for the plant and it is not a result of anything regarding nutrition, disease, pH, lighting or something that you have done. It is just a random occurrence and can be quite common in cannabis plants. It can not be prevented or remedied, it is nothing to be worried about and it will not affect growth rates, yields or potency.
It could be likened to how some people get freckles.
Great pictures of the exact problem too, a lot of people should take note!
Hope this helps, Organoman.