Can someone explain me this part please? if (numbers[i] != numbers[i-1] + 1)

Can someone please explain why the if-statement is (numbers[i] != numbers[i-1] + 1) and not (numbers[i] != numbers[i + 1])

        Console.WriteLine("Enter a few numbers, separated by a hyphen (eg. 1-2-3):");
        var input = Console.ReadLine();

        var numbers = new List<int>();
        foreach (var number in input.Split('-'))
            numbers.Add(Convert.ToInt32(number));

        numbers.Sort();

        var isConsecutive = true;

        for (var i = 1; i < numbers.Count; i++)
        {
            if (numbers[i] != numbers[i-1] + 1)
                isConsecutive = false;
            break;
        }

        var message = isConsecutive ? "Consecutive" : "Not consecutive";
        Console.WriteLine(message);