rx 480 8G after edit Wattman

I've 3 rx 480 8Gb. I'm using 16.8.2 driver
edited:

-16,5% frequency volt
Mv: 810 824 850 875 900 910 929
————————————————————
2200 Mhz with 920mv
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Temp target: 70 and max at 76•C
-25% power limit.
—----------------------------------------------------
It must be run at 27mh/s right ? But it's not. It's run about 23~26mh/s

It's show me:
Mining on Powhash 72.67 mh/s ( most )
........
Mining on PoWhash 62.44 mh/s ( some time )
Mining on PoWhash 83.06 mh/s ( Some time )


Ethermine.org show me that : 70~75.6 Mh/s ----
last 24 hours: 72 Mh/s <-- it's stop here.