These are all relative humidity, which is % moisture at a given temperature. So it is often 90% relative humidity outside in uk over winter, but that's air at 5degrees. The fan draws that 90% relative humidity air in through the slates, soffit vents or breather slates and pumps into house. As it flows in the temperature rises and therefore the relative humidity drops to 50 to 60%. Where as trickle vents, hit and miss vents and windows open just drop the temperature and therefore can raise relative humidity a bit. This is general and other factors are also involved.
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