¿Variation of altitude in ISS why?

Question

¿What's the cause that makes a variation of the altitude over the earth, in the orbit of the ISS ?

Answer 1

There are several reasons:

Long term altitude change is due to drag in the orbit caused by atomic nitrogen/oxygen/etc. The ISS has large solar and thermal arrays which actually bend due to drag. Drag causes the ISS to slow, decreasing the orbit. Periodically, the ISS performs a "re-boost" manuever to get it back up to a designated altitude above earth.

Short term, small altitude changes are also due to the fact that earth is not a perfect sphere, nor is the gravitational force the same for every position of the ISS. More on this in Wikipedia:
http://en.wikipedia.org/wiki/Gravity_%28Earth%29

Hope this helps!