Calculation of preamble and payload duration


Using the power calculator, I am trying to find the preamble duration and payload duration for the following configuration:

Channel = 2
Data rate = 6800
Preamble = 128
PRF = 64
bytes = 12

The following value I am getting:

Preamble duration| = 0,138.40 us
Payload duration = 0,040.00 us

But the verification portion of the power calculator is showing the different result for payload duration. Why is that? please see the attached image.

any idea why the results are different?

I think here is the explanation.

Thanks for the reply.