And by unstable I mean whatever you think unstable means.
In my opinion, the month of the Climate A is more unstable because if the record high is 20°C and the average high is 16°C that means that the high of some days is sometimes below the daily mean (15°C). The month of the Climate B has a high diurnal range but since the record high is 30°C and the average high is 25°C, it's most likely that the highs hardly ever drop below the daily mean (15°C).
Ditto for the record lows and average lows.
Agree or disagree?