XML : Fragment Layout not corresponding to preview

So, first of all I'm very new to Android so this may be a very dumb question, but..

I have in my mainActivity layout a viewPager which calls some fragments of different type (I have made 4-5 xml layout displaying different informations)

Despite I created all the layout basing on weighed LinearLayout to maximize compatibility, in the preview the visibility seems perfect in each device I selected, but once I load the application on my phone, the layout has bizarre behavior.

Here an example (preview vs. screenshot from my device)

http://s10.postimg.org/muf4q4cyx/Cattura.png

http://s10.postimg.org/b7v0ozpnt/Screenshot_20160326_165506.png

I'm not sure about where I made the mistake, I would like to know:

1) What happened

2) What cause this behaviour

3) How to fix it (a link with a guide would be very welcome)

Thanks very much!