The calculator merely calculates the values based on a perfect world scenario. It only says what the speeds would be if every component performed at their given ratings. Higher voltage doesn’t necessarily mean more torque or acceleration either. FWIW the calculator is pretty accurate to what I get on my board at 29mph (calculated 31mph).
At 12S though, there may be some losses in efficiency depending on your system, which can lead to lower speeds. @Photorph has gotten 38mph on his board which has a calculated speed of 41mph.
I’d say the calculator is pretty accurate for 10S and below.