The game checks the power and toughness of the creature that just entered as it exists on the battlefield. That might mean it's impossible to get another copy of that same card. For example, a Triskelion on the battlefield is 4/4, but a Triskelion in your library is 1/1. The power and toughness might also have been affected while the triggered ability was on the stack (such as with Giant Growth, Sudden Death, or Ovinize).