Arcane Subtraction 

Instant
Target creature gets -4/-0 until end of turn.
Learn. (You may reveal a Lesson card you own from outside the game and put it into your hand, or discard a card to draw a card.)
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Cards that do something similar expected a card that isn't here?
Reports become test cases the matcher is graded against before an update ships. Delvefall isn't a chatbot guessing, it compares the printed rules text of every card with plain math, and examples like yours decide which differences should count.
Obyra's Attendants // Desperate Parry 100%
$0.12#18898
"Target creature gets -4/-0 until end of turn."
Stream of Unconsciousness 99%
$0.22#18572
"Target creature gets -4/-0 until end of turn. If you control a Wizard, draw a card."
Chilling Trap 99%
$0.10#15074
"Target creature gets -4/-0 until end of turn. If you control a Wizard, draw a card."