A method that returns an Collection is returning an array when it's invoked by reflection?

There is a method in the Bukkit API: getOnlinePlayers():

public static Collection<? extends Player> getOnlinePlayers()
{
  return server.getOnlinePlayers();
}

In older versions of Bukkit it returns an array of Player[]; while in newer versions it returns a Collection<Player>.

I wanted to make it possible to have compatibility in both new and old versions, so I use reflection to invoke it:

public static Collection<Player> getOnlinePlayers(){
    Method m;
    Object obj = null;
    try
    {
        m = Bukkit.class.getMethod("getOnlinePlayers");
        obj = m.invoke(null, (Object[])null);
    }catch (...){ // omitted
        throw new RuntimeException(...); // omitted
    }

    if (obj instanceof Player[]){
        System.out.println("array"); // Used for testing
        return Arrays.asList((Player[])obj);
    }else if (obj instanceof Collection){
        System.out.println("collection"); // Used for testing
        return (Collection<Player>)obj;
    }else{
        throw new RuntimeException(...); // omitted
    }
}

Then here's the problem: When I tried this code in both new and old versions of Bukkit, it always prints "array". I am wondering why this would happen?