Humidity expressed as a percentage is a straight up percentage of moisture in the air. Humidity expressed as a wetbulb reading is an indication of how fast evaporation will occur, which will be quicker in drier conditions and slower in more humid conditions. Basically you're measuring the same thing but expressing it differently. but I don't think the huge differences in humidity used by some people are down to confusions between wetbulb readings and percentage humidity. People get success with a huge variety of humidities, and there's not really a one-figure-fits-all number the way there is with temperature.
If you incubate chicken eggs in 60% humidity days 1-18 then you're in the extreme minority. Either that or your hygrometer/wetbulb thermometer isn't accurate! (Seriously, have you calibrated your instruments? Always a good idea...) Most people using humidity that high would end up over-humidifying their eggs and drowning their chicks. Bearing that in mind, you should be careful of taking other peoples' advice about what humidities to use for duck eggs, cause what works for them may well absolutely not work for you at all.
Having said that, here are some numbers to think on...
Chicken eggs should lose 13% of their starting weight by lockdown. Waterfowl eggs should lose 15% of their starting weight by lockdown. While duck eggs need to lose slightly more moisture, they have a longer incubation period to do it in, so moisture loss rates will be roughly equal. Loads of people recommend that you incubate duck eggs in a much higher humidity than you use for chicken eggs, but I weigh all my hatching eggs to calculate correct moisture loss, and any time I've incubated duck eggs I've found that they actually require a very slightly lower humidity than what I usually use for chicken eggs.
However they DO require a higher humidity for lockdown and hatching. Most recommendations for ducks are 80%+ humidity for hatching, and I definitely noticed that the one time my humidity dropped to 75%, my last duckling struggled for quite a while to get out. I think duck egg membranes are a lot tougher than chicken eggs and can dry out more quickly.
Hope that info might be of some help to you!