That calculator is wrong.

Here's the way it works ...
The main OSC is running at 32768 hz.
The CPU is running a 8192 hz (32768 / 4).

Timer1 is loaded with the highbyte of 57344 and will overflow at 65536.
65536 - 57344 = 8192

So the timer overflows exactly 1.0000 seconds every time (depending on the crystal tolerance).