It is rather common, that Facebook conversion tracking and other tracking methodologies provide slightly differing results.
Attribution windows define how conversions are calculated and attributed back to clicks and ad impressions. Differences in attribution models or attribution windows (timeframe) can be a cause for differing figures on different tracking platforms.
Facebook and third-party tools (Google Analytics, Adjust) could be set up on different timezones, so when analyzing yesterday data you might get different numbers due to the difference in time (not the same midnight).
If the visitor clicks and views multiple ads, which ad gets the conversion?
Check how conversion attribution model is calculated in Facebook.
Also it doesn't make a difference if you have multiple ad accounts for the same website and page: the conversion is attributed to the ad that delivered the last click. If visitor didn't click any ad, only viewed many, Facebook attributes the conversion to the last impression.
3rd party metrics
Metrics from other measurement providers, like Google Analytics, DoubleClick and Adobe, or Mobile Measurement Partners like Adjust, Appsflyer, Kochava and Singular, have their own specific ways of measuring conversions and attributing them to ads.