Understanding backpressure in akka streams Source.queue

I'm trying out akka streams, but i can't get backpressure to work in my simple example. I am admittely not experienced with akka (streams), so probably I'm missing something big.

I am producing (offering on a queue) integers faster than consuming them, so I thought that backpressure would kick in. My goal is to always consume the most recent item that was put in the queue (that's why I have bufferSize = 1 and OverflowStrategy.dropHead() on the source queue).

public class SimpleStream {
    public static void main(String[] argv) throws InterruptedException {
        final ActorSystem system = ActorSystem.create("akka-streams");
        final Materializer materializer = ActorMaterializer.create(system);

        final Procedure<Integer> slowConsumer = (i) -> {
            System.out.println("consuming [" + i + "]");
            ThreadUtils.sleepQuietly(1000, TimeUnit.MILLISECONDS);
        };

        final SourceQueue<Integer> q = Sink
                .<Integer>foreach(slowConsumer)
                .runWith(Source.<Integer>queue(1, OverflowStrategy.dropHead()),  materializer);

        final AtomicInteger i = new AtomicInteger(0);
        final Thread t = new Thread(() -> {
            while (!Thread.currentThread().isInterrupted()) {
                int n = i.incrementAndGet(); 
                q.offer(n);
                System.out.println("produced: [" + n + "]");
                ThreadUtils.sleepQuietly(500, TimeUnit.MILLISECONDS);
            }
        });
        t.setName("ticking");
        t.start();

        // run some time... to observe the effects.
        ThreadUtils.sleepQuietly(1, TimeUnit.HOURS);
        t.interrupt();
        t.join();

        // eventually shutdown akka here...
    }
}

However this is the result:

produced: [1]
consuming [1]
produced: [2]
produced: [3]
consuming [2] <-- Expected to be consuming 3 here.
produced: [4]
produced: [5]
consuming [3] <-- Expected to be consuming 5 here.
produced: [6]
produced: [7]

Please ignore the threading stuff here and there just to fake getting data from an external source (like it would happen if I had to use this in a real project).

Any idea of what I'm missing?