Hi Jakub – 
 
I beleive this is to do with the TrueSKYLight within the scene giving additional lighting to your meshes (it acts very similarly to a skylight) – if your aiming to get the same results as the bottom image – you can either lower the diffuse multiplier which should return darker shadows overall (i found 0.1 or 0.05 seemed to return similar results to what you were looking for) , or fully disable the trueSKY light (untick the affects world)
 
Hope this helps 
 
Chris