Unequal side bands with a pure PM/FM signal using FFT

I have not used the FFT function in a long while, but when I tried to use the example code fromthe Help documentation, I ran into theoretical difficulties with the displayed spectrum. I have a pure PM signal input to the built-in FFT function, but it shows unequal side bands around the carrier. In my code below, I am using a 2 GHz carrier, 10 G Samples/sec samplling, with 21 MHz PM sinusoid. What am I missing?

Fs = 10e9;                    % Sampling frequency
T = 1/Fs;                     % Sample time
L = 100000;                     % Length of signal
t = (0:L-1)*T;                % Time vector
% Sum of a 50 Hz sinusoid and a 120 Hz sinusoid
x = 10*sin((2*pi*2e9*t) + 0.3*sin(2*pi*21e6*t)); 
y = x;

plot(Fs*t(1:500),y(1:500))
title('Signal Corrupted with Zero-Mean Random Noise')
xlabel('time (picoseconds)')
NFFT = 2^nextpow2(L); % Next power of 2 from length of y
Y = fft(y,NFFT)/L;
f = Fs/2*linspace(0,1,NFFT/2+1);

% Plot single-sided amplitude spectrum.
figure()
plot(f,2*abs(Y(1:NFFT/2+1))) 
title('Single-Sided Amplitude Spectrum of y(t)')
xlabel('Frequency (Hz)')
ylabel('|Y(f)|')