NogDog,
Strictly speaking, the e-ink displays in the Kindle and so on don't have the little black-and-white "balls" that you describe, this is an earlier technology called Gyricon (see
http://en.wikipedia.org/wiki/Electronic_paper). In the current displays each of those balls is replaced by a microcapsule which has charged particles of black and white pigment in it - under a magnetic field, the pigment moves to the top or the bottom so the microcapsule appears black or white.
The cause of the ghosting is just as you described, though - the pigment particles want to stay where they are so a full screen refresh has the flicker which is the screen pushing the particles top-bottom-top (or bottom-top-bottom) to get them moving. Without this refresh (as with the menus and so on) the particles tend to stay put and so the ghosting occurs.
Kuklachica, I found this ghosting quite irritating when I first started with my Kindle, but I soon got used to it and now never notice it. It is more noticeable with the menus, and with dictionary pop-ups etc - these are times when the Kindle doesn't do a full refresh (to make response quicker) and the ghosting is the result.
You should not normally see it from one page of the book to the next - the refresh (with flicker) between the pages should clear it. Occasionally I see it when going from a page with a picture to a page without, but not otherwise with text pages.
I would say the crucial way to test if there is a fault is that if you see ghosting from one page of text to another then this is not normal. Also, if you have ghosting on the screen and you press Alt-G which forces a full screen refresh (with the black/white flicker) and the ghosting goes away, then it is probably normal. If you do Alt-G and you can still see ghosting, then I'd be concerned that there is a fault and I would talk to Kindle CS.