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Iain
A very interesting question. The short answer is that it is caused by the fact that the eyepiece is not horizontal but at an angle. If you do the same thing with a big dob where the eyepiece sit on the side of the scope it sits at a different angle.
The critical thing is not to worry about it when using it for astronomical work. After all there is no up or down in space!!
When observing we quite often want to know what direction things are in relation to each other. For example if we find a bright star in the eyepiece and we know that a galaxy is 10 minutes to the east of it, how do we know where to look? For a newtonian like your scope a simple way is watch the star drift in the field of view, that tells you what direction west is. From that we now know east also and north always lies counter clockwise from west.
Cheers
Malcolm
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