Working differently on demo and live site $f = fopen( 'php://output'

Question

I am looking at the Yopp Polls plugin.

$f = fopen( 'php://output', 'w' )

As I understand it this line of code should mean it downloads to the local machine. On the demo site it downloads to my local machine but on the live site which is a multisite it downloads the CSV to an uploads folder on the server.

I've checked settings and can't see anything but feel like I'm probably missing something very simple.