Why don't modern CO2 records match direct measurments for the past 250 years?

It is very difficult to measure CO2 accurately. If you did it in a fairly crowded building, it would be several times the levels that are shown in places like the top of a mountain in Hawaii. That being said, I don't trust CO2 levels in ice cores as being accurate. It takes too long for the ice to become sealed. There are other problems like solubility and differentially forcing out certain molecules like you get in a reverse osmosis system. There are studies such as the stomata in leaves studies that shows that CO2 varies considerably more than ice cores suggest. Mostly the old tests weren't reliable. The new test, ice cores, that are used by alarmists isn't reliable either though.