Timings on Facebook ads

I wonder: When you change your time periods but keep your overall spend consistent, why does the daily estimated reach not stay consistent?

2 Days

4 days

Is it a thing where the rate at which they can predict the cost per person reached decreases the further ahead you project because it goes into an auctioning process?

I find it kind of annoying that you can't just find the answer to something like that with a quick Google search.

Instead it links to articles like this Kissmetrics article which pretty much just repeats the company line. For example I'm still not convinced that ultra specific targeting like that suggested in that article is effective.