I would imagine a combination of the following
1 - the moon does not orbit the earth in a "flat" orbit relative to the earths equator
2 - the earth's axis is tilted about 20 degrees
3 - the moon has a tilted axis relative to the earths equator
So the angle in which we view the moon depends on where the moon is around the earth etc
This will make more sense
http://www.youtube.com/watch?v=CswzrzqmvOA