Black Lotus Project relies on the pricelists that Magic Traders publishes every day for the pricing data shown throughout the site. They collect data from eBay and calculate the "price" by averaging the selling price of all auctions for a given card. Cards sold infrequently like Black Lotus or Time Vault have highly variable prices because there are so few auctions used to calculate the average. To compensate for the variability, Magic Traders publishes two versions of the physical Magic card pricelist: a monthly version and a weekly one.
The monthly pricelist uses prices from the past 30 days of auctions to calculate the price. Since prices for odler cards change slowly, this list gives an accurate price for cards that have been in print for a while like the two examples above. The weekly list often does not have enough data to show any price at all for cards that are sold infrequently.
The weekly pricelist uses prices from only the past 7 days of auctions to come up with its numbers. For newer cards and cards whose popularity can spike overnight like Grove of the Burnwillows, the weekly pricelist gives a far more accurate price than the monthly one can. The older values in the monthly list weigh down the average price, causing BLP to sometimes show a much lower price for a card than it should.
I am looking at an intelligent way to store and display the weekly versus monthly price for all cards so you can rely on BLP for current trading values. Cards will have separate graphs for the two pricelist types, and the daily graphs will likely use the weekly list instead of the current monthly one.