The changes in propagation can be enormous. The SNR gives a good indication of the strength of the WSPR spot.
A WSPR spot with a SNR of -29 dB is a solid copy.
The stronger the signal, the higher the SNR will be.
With a SNR of -19 dB, the signal is 10 dB stronger.
With a SNR of -9 dB, the received signal is even 20 dB stronger, than necessary for a solid copy.
The line of 200 mW in the table shows, that when the SNR is -19 dB, you can reduce your power from 200 mW down to 20 mW and still be spotted. The spot will have a SNR of -29 dB. So the “Calculated lowest possible power” of this spot is 20 mW.
When the SNR is -9 dB, you can even be spotted with 2 mW. So the calculated lowest possible power of this spot is 2 mW. WOW