On my boat I've seen a best of 70mph with light load and choppy water.
In a couple of occasions with similar load, but with completely flat water, I've not been able to go above 66/67mph.
Can such loss of speed make sense for the different water conditions alone, or is that a coincidence and there must be some other reason I'm not aware of?
Thanks for your views!

yes. the chop airates the bottom like steps.

Most boats benefit from "getting some air under them." Generally speaking, air has a lot less drag than water. So, it's not at all unusual to gain some speed when conditions are not glass smooth. You'll see it mentioned in Powerboat Magazine many times where their testers thought a particular hull had more speed in it, but the flat water held it back.

Another thing tends to come with glass flat water many times though - humidity. Your engine will not perform as well in hot, humid air as it will in cool dry air - again - generally speaking. So, you might have actually had two things working against you.

Normal. My boat will run about 74mph in a 1ft chop vs 72mph on dead flat water.

Same reason they put steps on boats.
Get some air under them.

I loose at least three or four mph in dead flat water. Its commonly known as sticky water

Thanks folks, probably I wasn't clear enough in my question, I'm already aware of the aeration effect in principle, but it was the amount of it that I was wondering about: 3 or 4 mph out of 70 seemed a lot...
I never thought of humidity, though. Two Stroked raises a good point there.
And I'm also above sea level, btw, though not a lot - 650 feet or so. Does anyone have an idea of how much that can affect the performances?

just like stated above different boats loose different speeds, my baja 24ol would run 80 on the river flat water but a mile out in the bay it would run 3 to 4 mph faster with 1 to 2 1/2 ft chop maybe more but I ran out of prop back then.

There are standard correction factors used on engine dynamometers that allow for things like changes in humidity. Maybe one of our other members has that correction data handy?

Salt water vs fresh water also can add 1 or 2 mph.