When comparing the stats as Sharp-Eyed Rookie's ability resolves, it's possible that the stat that's greater changes from power to toughness or vice versa. If this happens, the ability will still resolve and you'll put a +1/+1 counter on Sharp-Eyed Rookie and investigate. For example, if you control a 2/2 Sharp-Eyed Rookie and a 1/3 creature enters the battlefield under your control, its toughness is greater, so Sharp-Eyed Rookie's triggered ability will trigger. In response, the 1/3 creature gets +2/-2. When Sharp-Eyed Rookie's ability tries to resolve, the other creature's power is greater, so you'll put a +1/+1 counter on Sharp-Eyed Rookie and investigate.