Behaviour of table.remove()

[Марк Гуревич <mark_gurevich@...>]

Hello, everyone!
Could somebody explain me the behaviour of table.remove() in the following code:

function f(t)
    print(t[1], t[2])
    table.remove(t,1)
    print(t[1], t[2])
    table.remove(t,1)
    print(t[1], t[2])
end
f{1,nil,3}
f{1,nil,3,4}

The output of f{1,nil,3} is
1       nil
nil     3
nil     3

and the output of f{1,nil,3,4} is
1       nil
nil     3
3       4

Why does the second call of table.remove() leave nil in t for f{1,nil,3} and remove nil from t for f{1,nil,3,4} ?