GH3 = attempt at Photographic Dynamic Range

Since ISO 25600 is extended there is no way you can get the same DR for ISO 12800 as for 25600. DR at 25600 will be about 1 stop less. If your result is different you have severe error in methodology.

And I suspect ISO above 3200 will already be extended so with proper measurement you should get a chart similar to DxO chart for E-PM2. It's the same sensor after all.

BTW, according to manual you posted, GH3 has no real ISO 125, it's overexposed ISO 200, just like Canon has "ISO 50" on 5DMkII. So, unlike your chart shows, ISO 125 should have the same DR as ISO 200.

Bottom line - I agree with you, you probably did something wrong.