My best guess was that seven is one of the more common numbers for card games where you draw cards and perhaps it won out because it was what Richard Garfield thought people would assume to draw
Gren Maju Da Eiza , Golden Homunculus , and Helios The Primordial Sun all gain ATK and DEF according to the number of cards banished
This is entirely due to the new Elemental HERO Shining Neos Wingman (YGOrg Translation) , a Level 8 Fusion that destroys cards your opponent controls up to the number of different attributes on the field
Maybe hybrid mana causes problems in rules text, and they didn't want to make adjustments for a single digital-only card
C Common The 13th Grave X DARK Level 3 [ ZombieNormal ] ATK 1200 DEF 900 A zombie that suddenly appeared from plot #13 - an empty grave
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