there is always information about input and output interface in flows. So even if you export data only from 4 interfaces, you can have other interfaces involved in flows as input or output interface.
Do you think that this could be the reason?
Errr : maybe?
What you say sounds plausable to me but as I mention I'm a Netflow newbie. Still learning the in's and out's of the Netflow\SFlow protocol.
To prove that, you can analyse exported data from node by Wireshark or Microsoft Network Monitor.
You will see which data you export. If you are not sure, you can post that pcap here and I will look at it.