Power mismatch with theory

Hello everyone

Suppose I plot a chirp as follows:

As can be seen the amplitude is 0.632v peak-to-peak that corresponds to 0dbm.

After I take the power spectrum of this signal, it is not 0dbm. Could anyone describe why this mis-match happens.

I tried on a simple sinusoidal signal and that works because I get a 0dbm output for a 0.632v peak to peak signal, but for a chirp there is a mismatch between calculations and what I see on matlab screen.

Below is my function for chirp generation:

And I use below code to generate the above generated chirp:

fs=500; %sampling frequency

t=0:1/fs:1; %time base - upto 1 second

f0=1;% starting frequency of the chirp

f1=fs/20; %frequency of the chirp at t1=1 second

x = mychirp(t,f0,1,f1);

plot(t,x);

xlabel('Time(s)');

ylabel('Amplitude (v)');