Confused about humidity levels in basement - a math problem

I can't work out the math on this one.

I have a basement. The size is about 10,000 cubic feet (about 35 by 32 by 9). I bought a dehumidifier that says the humidity is 70 %. I want to get this below 50 %. The temperature in the basement is about 70 degrees (Fahrenheit). Now, here is where I may be doing something wrong.

at 70 degrees 100 % each cubic foot of air should contain about 0.0011 lb of water. I get that from here

https://www.engineeringtoolbox.com/maximum-moisture-content-air-d_1403.html

So at 70 % there should be about 0.00077 lbs of water per cubic foot. At 10,000 cubic feet this would be 7.7 lbs of water in the air in the basement. Since I started running the dehumidifier yesterday, the humidity has dropped to 65% (so not a lot). In that time (so about 24 hours), the dehumidifier has produced about 5 gallons of water. But five gallons of water has a weight of 42 lbs.

And that is what I don't understand. If there is only 7.7 lb of water in the air, how can the dehumidifier take out 42 pounds of water and only lower the humidity by 5%?

The basement is sealed of pretty good. There is a crawl space covered in plastic. It doesn't smell bad and I was a bit surprised with the 70% humidity. I thought it was less. The numbers above are very rounded, but 7.7 lbs is nowhere near 42 lbs.

• When I start my dehumidifier it slowly drops the humidity level. It takes several days for it to go down 15% - 20%. In the first day 5% seems about right. May 21, 2021 at 3:31
• @SteveSether: You got me thinking a bit more. Does this mean that it is feasible for me to dry out the walls? The basement really is sealed off from air pretty well. May 23, 2021 at 20:51