为什么这会改变这个类中所有这些变量的所有实例？

public class MyClass {
    private static int staticVariable = 0;
    private int instanceVariable = 0;

    public void modifyVariables() {
        staticVariable = 10;
        instanceVariable = 20;
    }

    public void printVariables() {
        System.out.println("Static variable: " + staticVariable);
        System.out.println("Instance variable: " + instanceVariable);
    }

    public static void main(String[] args) {
        MyClass instance1 = new MyClass();
        MyClass instance2 = new MyClass();

        instance1.modifyVariables();
        instance1.printVariables(); // 输出：Static variable: 10, Instance variable: 20
        instance2.printVariables(); // 输出：Static variable: 10, Instance variable: 0
    }
}