observeOn vs subscribeOn

public static void main(String[] args) throws InterruptedException {

        Observable.zip(stringObservable().subscribeOn(Schedulers.io()),
                integerObservable().subscribeOn(Schedulers.io()),
                (s, integer) -> {
                    timedLog("Got items, zipping");
                    return s + " " + String.valueOf(integer);
                })
                .subscribeOn(Schedulers.io())
                .observeOn(Schedulers.computation())
                .subscribe(zipped -> timedLog("Got zipped value + " + zipped));

        Thread.sleep(5000);
    }

    public static Observable<String> stringObservable() {
        return Observable.defer(() -> Observable.fromCallable(Main::getStringItem));
    }

    public static Observable<Integer> integerObservable() {
        return Observable.defer(() -> Observable.fromCallable(Main::getIntegerItem));
    }

    public static String getStringItem() {
        timedLog("Start of getString item");
        try {
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        timedLog("Returning string item");
        return "String item";
    }

    public static Integer getIntegerItem() {
        timedLog("Start of getInteger item");
        try {
            Thread.sleep(1500);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        timedLog("Returning integer item");
        return 100;
    }

    private static void timedLog(final String message) {
        final long now = System.currentTimeMillis();
        System.out.println(now + " - " + message);
    }