What is the variation like within species? Have they all lost ~7% or have some lost 3 and some 10? Are they all candled fertile or do you have some duds which might skew the average?
Also specially, are they grouped in breed/species? is one side where the ducks of one breed are more humid than the middle or other sides?
My incubator has cool and dryer areas. After trusting a single thermometer with my first hatch, I still hatched 6 of 12 at 35'C, though they were several days late, they did grow into good laying birds. So I don't worry quite so much about keeping everything to the book. Though I now calibrate against another mercury and keep a digital in as well.
I also wouldn't expect duck and chicken eggs to behave in the same way under the same conditions. And I guess your extra grit seems to be a reasonable explanation for the difference between duck breeds.
What to do..? I'm not sure. I guess the obvious would be as you suggest to up the humidity for the hens a little and maybe reduce for the ducks a touch to try to create conditions that would be a compromise for both breeds?
Not enough humidity is definitely bad for hens eggs at the end, as I have seen them if the airsac membrane dries and shrinks before or during pipping. I've not worried so much about humidity through hatch but up it to max with a highest airflow also for hatching day. I do this by opening all vents and standing incubator in a tray of shallow water as well as filling up the water pans. Seems to work well, for eggs that have not travelled, I had 90%+ hatches last year.