Average Hashrate Questions

Can anyone comment on the average hashrate ("KSol/S(avg)") that the Z9-mini reports (not pool-side hashrate)? Specifically what I'm wondering is average since when? Since boot time? In last 24 hours? In last hour?

The reason I ask is because I've been playing with Frequency setting, and getting some strange results. For example, I ran on Frequency 750 and after 30 minutes got an average of 15.45 kh/sec average. Then I ran on 737 for four hours and got 16.15 kh/sec average. After rebooting because of a circuit breaker tripping, I'm now averaging 15.98 kh/sec on 737 after two hours.

Anyone else seeing these kind of strange results? Am I just not waiting long enough for the average to even out? If so, what's a good amount of time to wait? Or could really be the case that my mini hashes less on 750 than 737?