Why Do Star Trails Appear Denser on One Side of the National Geographic Photo?

[kuahji]

National Geographic is featuring this star trail picture this week on their wallpaper page (found here http://ngm.nationalgeographic.com/your-shot/weekly-wrapper/2012/img/0812wallpaper-week-4-1_1600.jpg). In the picture though I was wondering why the star trails appear so much more dense on the right side of the picture than the left. Any ideas?

 

[Drakkith]

That section of the sky simply has more bright stars than the other.

 

[NeuronsAtWork]

I'm sure Drakkith is correct--also the actual trails seem a bit overexposed, and that exacerbates the effect. I would guess that the photographer possibly 'stacked' multiple images and the alignment might have been off a bit, because the single exposure shots I've seen tend to be sharper. Anyway, see the shot on the homepage here--
http://startrails.de/
It resembles the one you referenced, as far as the imbalance you noted...