PCB Antenna Width

I am looking over the stack-up for the dev board which I received from an FAE and has designed in a 50mil width PCB antenna. I am using this online calculator here:

emclab.mst.edu/.../index.html

From this calculator and based on the board specs I am not seeing 50mil being a 50-Ohm trace with the given PCB stack-up. Can you explain why/how Nordic arrived at 50 mil with the attached stack-up? I am not sure why my calculations aren't matching Nordic's.

4layerstackup.PNG

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