Spectral analysis of spectrum analyzer
So here are a few hints;

If you account for the 20 dB attenuation, you get a number that is much closer to your estimate, but off by a factor of 2.

(The picture of your spectrum analyzer says "Att: 20 dB" on the top.)

To resolve that factor of 2, you could try using the mean amplitude instead of the max amplitude in your estimation of the power at the first peak.