I've noticed that the D&D bundle doesn't include the Forgotten Realms: The Archives - Collection One (but does include 2 and 3), where as previews sales bundles have included all three.
Is this an oversight or more simply a quirk of the sale this time around?

I've noticed this as well.
Hard to say what the truth is, only GOG itself knows, but I think it's a deliberate choice because the bundle contains two out of three archives of the "Forgotten Realms" collection and two out of three archives of the "Dungeons & Dragons" collection: both "Forgotten Realms: The Archives - Collection One" and "Dungeons & Dragons: Krynn Series" are missing (not counting "Dungeons & Dragons: Dragonshard").

I noticed that as well, and in fact there is no discount at all on Collection One (even if you wanted to purchase it individually)! Since nearly all of the games are discounted, that makes me wonder whether it might be an oversight...or perhaps an indication of a one-day deal on Collection One coming after today?