I believe I figured out that the placeholder card for planeswalkers takes the place of a land in Origins packs, so my count here should be 284.

After scratching my head trying to figure out why the count was only 279, I found the 5 white creature cards listed here as Sideboard sitting right in front of me, but not with the rest of the box.  So, I'm thinking they just got separated from the rest; also thought it was odd that I only had 8 white creatures when I was counting initially.  If I discover I'm wrong later (doubtful), I will remove them immediately.

NOTE:
Deckbox currently lists the "last important" set that the card was in, instead of the one it was actually identified as in the inventory.  This is fine, but updating that feature for Origins has lagged on some cards, so a few listed here may display the wrong set.  Please refer to my inventory for the real set that the card is from.

For my packs as I post them, I will mark which set the pack is from in the notes, as I'm doing here.