Sound input read/sound input configure

Hello to all readers, I have a burning question regarding an example program I changed a bit; 

I read 9.000 samples/ch in the sound input config.

For some reason, before reading the input, this number gets halved, and I don't know what the significance of this is, why have 9.000 samples in the buffer if you're only going to read 4.500? 

Thanks in advance.

 LabVIEW version 2014 Student edition