Because that's how the projections are built. They look at pre-release marketing numbers, advanced ticket sales, number of screens, and comparable releases, alongside variables such as star power, director track record, franchise, etc. It's not just some dude saying "I think this movie should gross X this weekend" it's based on predictive analytics. Here's a paper on some of the techniques for developing these projections from 2012 that you may want to look at.
As to assuring you about the disappointment the studio is feeling, all you have to do is look at the quotes they're giving the trade papers about the performance from the weekend.