While we can send all your conversions to the Facebook Pixel it does not mean that Facebook can attribute all conversion data to Facebook users. You need to consider that not all your website visitors necessarily have a Facebook account, and if they do their session did not necessarily contain a Facebook cookie or FBCLID if they did not come directly from Facebook to your website. While we also send along user agents and (anonymised) IPs, these two values often are not enough for Facebook to do a proper attribution.