Noticed a +$100 value change during a period of time I haven't accumulated any new cards.
Sorted by price, two Plains valued at ~$50 were at the top of the list.
I definitely don't own these cards. Nor have I ever. Nor would I have added them, as the majority of my lands are Post-2013.
The site also claims the two lands were added to my collection over a month ago.
I checked the value this past week, and it was at the correct value then. Not the +$100.

So uhm. What the heck happened, is my question.

EDIT:
SCG price lists these Plains as 50 cent cards. Something is messed up with the Deckbox's pricing system.