Yes, I saw it some days ago, and it surprised me too. When communities started the featured image was a random image of the hot 20, now this has changed and the featured image is one image and in the communities that I've seen this image isn't of the hot 20. I suppose that to put an image of the hot20 was discarded because this requires more time to process; this is to process the hot 20 and the random selection. Anyway the randomly selection of pre-selected images with votes is the most representative way. Of course the work of moderators is important for the main page of a community. For example in the community of photography every month the moderators select the featured gallery of the month and the featured photography in the main page is, during the month, a selected image of this gallery.