The nearby system was originally supposed to show you pokemon spawns in order of distance (Which we all know didn't work at all). Essentially Sightings is now a list of what pokemon are currently in the area as opposed to listing every nearby spawn as an individual pokemon, which also caused a bunch of duplicates to clutter the tab. They also took out the ability to "watch" a pokemon since that was based on distance as well. Sightings doesn't appear to sort pokemon in any particular order, at least not alphabetically or numerically based on pokedex number