Why does a random double produce different results

I was taking an online java course and given this question, my idea was to generate 2 random double(s) and subtract one from the other as each will produce a double between 0 and 1.

This program is to compute an estimate of pi by simulating dart throws onto a square.

The instructor answer was to multiply the generated double by 2 and subtract 1.

My question is, where those two ways differ?

Below is the instructor's answer:

public class MonteCarlo
{
public static void main(String[] args)
{
    System.out.println("Number of tries");
    Random generator = new Random(42);
    Scanner in = new Scanner(System.in);
    int tries = in.nextInt();

    int hits = 0;
    for (int i = 1; i <= tries; i++)
    {
        // Generate two random numbers between -1 and 1

        double x = generator.nextDouble() * 2 -1 ;

        double y = generator.nextDouble() * 2 -1 ;

        // Check whether the point lies in the unit circle

        if (Math.sqrt((x - 0) * (x - 0) + (y - 0) * (y - 0)) <= 1)
        {
            hits++;
        }
    }

    /*
       The ratio hits / tries is approximately the same as the ratio
       circle area / square area = pi / 4
    */

    double piEstimate = 4.0 * hits / tries;
    System.out.println("Estimate for pi: " + piEstimate);
}
}