Consider this, even if his ph tester was off, if he uses the same tester for his tank, then outside, even being off there would be a difference, and he's not seeing one.Ok, I didn't know that, but the average hobbyist didn't use them.
And the outside co2 levels would play a part. But it is not preventing his pH from being above 8.0.
You don't know the accuracy of his measurement,,, or even the co2 levels specific to his exact location. There are other factors besides co2 that determine overall pH values, & as bertoni said "It could be a measurement issue. That'd be my first guess. Either the pH measurement or the alkalinity could be the issue, with pH measurement issues being more likely. Outside aeration should get the pH beyond 8.0 after a few hours".
My pH is constantly 8.2 to 8.3 & I live on the same planet as nemmy.
He's pulling fresh air in from outside. If that air is limiting the pH level, then yes the outside air is a contributing factor. If inside his pH was say 7.8, then the same water outside is 8.3, then I would say there is something inside causing this. But it's the same in or out.
Same planet, correct, your sig says you are from down under, is there much manufacturing near you? The OP stated "I just at this moment remembered there's a coal burning plant about 1 mile from my house".
What is your outside Co2 level?