android - Action bar tabs showing as drop-down list

Question

I'm not able to understand this behaviour.

I have an activity with fixed tabs layout, but:
in landscape the tabs are shown as a list, whule in portrait the tabs are shown as I want.

I have read about split action bar but I don't think is the right solution.

Any idea?

EDIT

Here the activity with pager:

<android.support.v4.view.ViewPager xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:id="@+id/pager"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context=".CapsulesActivity" />

And here the fragment layout:

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:paddingBottom="@dimen/activity_vertical_margin"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
tools:context=".CapsulesActivity$DummySectionFragment" >

<GridView
    android:id="@+id/gridview"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:columnWidth="290dp"
    android:gravity="center"
    android:horizontalSpacing="20dp"
    android:numColumns="auto_fit"
    android:stretchMode="columnWidth"
    android:verticalSpacing="20dp" >
</GridView>

I also post images for portrait and landscape of my app.