Naturally the blazer is going to have lighter trousers, as it's so dark. I wouldn't read too much into that.
Camel hair seems to be out of fashion, but I imagine there's a reason you still see 'em.
Always opting for lighter trousers seems very limiting. An outfit where the overall effect is dark -- say medium brown coat with dark brown cords -- can make for a very rich look.
Aside from that, some guys' complexions benefit from a very light coat.
Brown is something of a special case in that you wear it together in different shades.
There is more flexibility with a lighter grey trou because if gives a greater range of contrast.