The ability checks that creature's power only once: when that creature enters. The trigger checks a creature's initial power upon being put on the battlefield, so it will take into account counters that it enters with and static abilities that may give it a continuous power boost once it's on the battlefield (such as the one on Glorious Anthem). After the creature is already on the battlefield, boosting its power with a spell (such as Giant Growth), activated ability, or triggered ability won't allow this ability to trigger; it's too late by then. Once the ability triggers, it will resolve no matter what the creature's power may become while the ability is on the stack.
Format Legality
Standard
Not Legal
Future
Not Legal
Historic
Not Legal
Timeless
Not Legal
Gladiator
Not Legal
Pioneer
Not Legal
Modern
Legal
Legacy
Legal
Pauper
Not Legal
Vintage
Legal
Penny
Legal
Commander
Legal
Oathbreaker
Legal
Standardbrawl
Not Legal
Brawl
Not Legal
Alchemy
Not Legal
Paupercommander
Not Legal
Duel
Legal
Oldschool
Not Legal
Premodern
Not Legal
Predh
Legal
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