I deliberately stayed away from claiming a cause in my post, because I don't know the cause. I only know the result of the system as a whole; every cell phone camera I've had in the last 10 years does not actually take pictures at its claimed resolution, even in broad daylight. (I can also say it can't solely be over-aggressive JPG-ification, because the result doesn't match that; the 'blobs' I refer to cross the JPEG DCT boundaries freely. I can't say bad JPEG-ification isn't part of the problem, though.)

I tend to buy last year's midgrade phone because I don't really need the latest and greatest. Maybe the top end really does take the full photos they claim, but that still leaves a lot of cell phones that are lying.

Most of the recent cell phones take perfectly adequate pictures, considering the amazing disadvantages they are laboring under. It's just that if you downsample the pictures to 2 or 4MP, you're basically losing no information.

(There are similar effects with video. The aforementioned 10+ year-old camera I had suffers from this as well. It has a "720P" and a "1080P" video mode, but based on my experimentation, both modes get real video out at about 400-500 "real" lines of vertical quality. I can squeeze either video's source down to that size and when I compare the full screen result to the original, I can find no difference whatsoever. I imagine more modern real cameras probably do a better job here; this thing is pretty old now, obviously.)

Not very.