Why is the nucleus accumbens in that place?
The ventricle looks in a different place simply because the first image is a transverse section whereas the second one is a sagittal section. 
In the sagittal version the striatum looks very far from the ventricle because it is a quite lateral slide, more medially it would look closer to it as it does in the first figure you show.

Here you can see a representation showing also a more medial transverse section.
source: Vertebrate brains and evolutionary connectomics: on the origins of the mammalian ‘neocortex’. Harvey J. Karten. (2015) DOI: 10.1098/rstb.2015.0060