request.headers.referrer_host in Bulk API

Hi, 

in many cases, I found some visits where the first step (sorted by event.time.ms) has a request.headers.referrer_host =  my own community hostname.

I would expect to find 'null' for direct traffic, or another hostname (google or any referrer site), but I don"t understand how the visit source can be by own site.

1 ) How is it possible ? 

2) I noticed that in Community Analytics, in the referrers section, my hostname is not listed in the Referral Domain, although it appears in Bulk API ==> why it is filtered out ?

thanks