That sounds normal to me. I've tested mine out in less than 5 days or up to 12. I would have thought they'd test out around the same time but apparently not. My RE said it's totally normal and not to worry. It doesn't mean anything in terms of getting a BFP. Now you know that if you get a positive it's for real. GL!
ETA: The earliest you could get a BFP is 9-10 days past IUI. That would still be very early. Wait until 12-14 dp IUI and then consider that accurate. You haven't even had chance for an embie to implant yet.