How does Siri come up with contact suggestions?
Relevant people
As Apple specified on its website page dedicated to iOS 9, with your Siri suggestions you can
See the contacts you talk with most, those you talked to last, or even those you’re next scheduled to meet
I'd definitely suggest you to have a look to all that Siri suggestions include from here, in fact it doesn't just suggest you Contacts, but also
- Places
- Apps
- News
Hope this helped you!