"Unknown contact" displayed in the report "Camp Analysis Overview by Forms - Total Form Submissi..."

Hi! I see a group of contacts that are displayed as "Unknown Contacts" in Eloqua Insight reporting - see attachment.
Can you please let me know what the possible causes might be?

I already excluded the possibility that these contacts are deleted since I see the same number of contacts used in the segment campaign.

So is it possible that the "unknown contacts" are the result of emails-forwarded or the fact they did not accept the tracking cookie ? I fund this article but I do not think it gives a detailed explanation.

Best regards,

MM