Probably the desktop app pre-process the image with its own renderer using dithering algorithms to give it a smoother finish. Which actually is reasonable - those images don't change frequently enough so a bit overhead when adding them to desktop is quite acceptable - and since those are the images that users will most frequently see there is a reason to do that from a marketing/general user experience point of view.

You can see a City Bank written on a bus, that doesn't mean that you'll get any money if you enter it. iPhone has way worse screen than the N900 - color depth is the same, N900 has 2.5 more pixels thus resulting in far crisper image and especially text, and the N900 has better viewing angles (actually I'm quite surprised how good it is at that given that it uses TN-Film which by itself has abysmal viewing angles which only get worse as the dpi rises, and N900 probably has the highest DPI I've ever seen on an LCD).

Oops , I stand corrected. I forgot to consider that for the sake of color uniformity as the debate started on white-to-black gradient where additional bit to any of the colors plays no role except giving it a hardly noticeable tint. Yep, when limiting a screen to the 16-bit (or any other depth indivisible by three) you need to give an advantage to one of the colors to have 65k colors available, but when considering a grayscale we can safely say that the screen actually performs as 15-bit, which is precisely the reason why there are 32 distinguishable bars on a full white-to-black gradient.