Oh, I figured why you ask "how do you come up with 2.74 miles". I thought you knew this, sorry.
The phenomenon is called Fata Morgana. The visual horizon appears to be way behind the platforms. But the physical horizon isn't there, it's a different thing, it stands at 2.74 miles from the observer. You would need an ass long stick to measure it physically, your eyes won't help measuring it.