That's trademark Bungie style. They may not always hit to the highest graphic fidelity but it is mostly because of their obsession with larger maps and a colorful world (against likes of Gears or Kill Zone).
It's also because they're doing last gen and next gen...I don't expect those releases to be vastly different on the varied consoles.
Seeing as both next gen consoles are x86 based and the nature of the OS on the Xbox one, porting would be a lot less difficult across platforms than the current gen.
True for coding, but you still have to deal with legacy OS/32 bit OS support (or hopefully they stop dealing with them,) average spec machines (which are claimed to be quite poor, who knows) and a lot more hardware variety.