If you're advertising your events on Facebook, it's normal to occasionally notice a discrepancy in the conversions reported in your Facebook account compared to the conversions in your Crowdcast event analytics.
The main reason for this is that Crowdcast tracks the lead's source by the HTTP referrer header value while Facebook ties conversions to their Facebook account.
Cross-device attribution: If a user clicks a Facebook ad for your event on their desktop browser then lands on Crowdcast anonymously (they are not signed in to their Crowdcast account) then Facebook and Crowdcast will each register 1 new lead from Facebook. Later the user might come back to the Crowdcast event in another browser from a different source. Since the user is in a new browser and did not come from Facebook then Crowdcast will track this as a new lead. If the user registers then Crowdcast will count this second lead as converted and the first lead as unconverted. Facebook will count the first lead as converted.
Clicks vs. impressions: If a user sees your ad on Facebook but doesn't click but later comes and registers for your Crowdcast event, Facebook will count this as a conversion but Crowdcast will not attribute the conversion to Facebook because the user didn't click the ad.
These are just a few examples that could cause a discrepancy between conversions reported in Facebook and Crowdcast. This Facebook help article offers an in-depth explanation of other reasons reports may not match.