I remember hearing this is the correct way to interpret some weather forecasts. 90% rain could mean certain rain over 90% of the area, rather than a 9/10 chance of rain over all the area
This is correct, or at least was as of 25 years ago when the question came up in conversation with a professor of mine, I asked on Usenet, and got a response from someone who worked for the National Weather Service.
Apparently they had specifically debated about whether to weight it on population density. In other words should they try to make it be so that a 90% chance of rain means that 90% of the people listening to the forecast encounter rain? They decided not to on the principle that people do not care equally about forecasts. Farmers care more than city dwellers, and city dwellers who care about forecasts pay more attention when they are deciding whether to take a trip to the country. Sorting out all of these issues to produce a perfect forecast for the audience was a can of worms that they decided not to dive into, and instead they kept to the simple, unambiguous and easily measured percentage of land area.