My GP referred me to a clinic who just happened to be a bulk billing practice. First time I had ever heard of that! She said a certain amount of scans are bulk billable although not the NT scan. I am quite stoked to have found this clinic as they said if I have referrals from a specialist (being my ob) they can bulk bill nearly all my scans. I need quite a few to keep an eye on my cervix so I am cheering! Last time it cost us a fortune for all the scans. Worth it of course
So, in answer to your question I have no idea why some can bulk bill and some don't!