I think the "Every 5 seconds" shown speed or the average 5 min. are wrong.
With my 2 x RX580 & -clKernel 0
I have been looking carefully for some 15 min. periods, at the screen I see:

70 % of time 64+ MH/s (rarely up to 65)
15 % 62 MH/s
15 % 58.1-58.2

The average 5 min. shown is 57.9 to 58 after 15 min., lowest than the lowest 5 seconds speed.
Also the average goes lower within time while the 5 sec. are the same.
I think the same happens with Kernel 1 but differences are small & it is harder to know for sure, I have not tested it carefully yet.

Looks like the 5 min. average is metering the lowest H/s or I don't know what.

More:

I checked it again this time for more than 30 min.

The lowest 5 sec. have never been below 58.15 (Most of time between 58.2+ & 64.7+) while the highest average 5 min. have never reached 58.1 MH/s (58.08 at most, usually below 58).
So, something is not OK.