The magnitude of the variation isn't nearly the same in the O2 vs CO2 cases. "16% O2 reduction is lost in the noise" is devastating evidence against the theory "0.2% O2 reduction has significant cognitive effects", but "16% CO2 reduction is lost in the noise" is weaker evidence against the theory "66% and 300% CO2 increases have significant cognitive effects".
I'm not arguing with you about implausible effect sizes, though. We should especially see significant seasonal effects in every climate where people typically seal up buildings against the cold or the heat for months at a time.
One or two research groups have published work on carbon dioxide and cognition. The state of the published literature is confusing.
Here is one paper on the topic. The authors investigate a proprietary cognitive benchmark, and experimentally manipulate carbon dioxide levels (without affecting other measures of air quality). They find implausibly large effects from increased carbon dioxide concentrations.
If the reported effects are real and the suggested interpretation is correct, I think it would be a big deal. To put this in perspective, carbon dioxide concentrations in my room vary between 500 and 1500 ppm depending on whether I open the windows. The experiment reports on cognitive effects for moving from 600 and 1000 ppm, and finds significant effects compared to interindividual differences.
I haven't spent much time looking into this (maybe 30 minutes, and another 30 minutes to write this post). I expect that if we spent some time looking into indoor CO2 we could have a much better sense of what was going on, by some combination of better literature review, discussion with experts, looking into the benchmark they used, and just generally thinking about it.
So, here's a proposal:
Some clarifications:
(Thanks to Andrew Critch for mentioning these results to me and Jessica Taylor for lending me a CO2 monitor so that I could see variability in indoor CO2 levels. I apologize for deliberately not doing my homework on this post.)