thanks for taking the time to ask this not so trivial question! A deviation of almost 4x sounds like quite a lot indeed. Let me break down why there is any difference in the first place:
In a nutshell, there are different levels of engagement Facebook considers as “clicks” when it comes to ads. The highest one just tracks every interaction with the ad. That includes likes, comments, shares, profile clicks and finally external link clicks. At the moment we are getting “all clicks” from Facebook, meaning all interactions. But we are considering to switch to “link clicks only” as it will be closer to the metric that most users will finally be interested in, which is landing page visits, and thus will be less confusing. But even when we do this, there will always be a gap between Facebook clicks and landing page visits because users may wander off in-between clicking and actually visiting the page.
Please let us know if you have more questions about this.