Because the variable was assigned in the timeframe captured by t. It's an example of time travel to the past. The semantic is confusing at first but think of

Because the variable was assigned in the timeframe captured by t. It's an example of time travel to the past. The semantic is confusing at first but think of it like now() took a snapshot of your program and "at t" went back and modified it. The compiler will evaluate the entirety of your program before execution, and sequence things as directed. If you're wary this could lead to incomprehensible programs, remember: It should be understood only as an exploratory game.