[egg]

Ideally, when done the beer will have more CO2 than is in air and no O2 at all. So left open to the air, O2 will equalize by entering the beer and CO2 will equalize by leaving the beer.

Pressures not percentages... beer will never be 70% nitrogen.

OK, I think I get that.  So while there may be an overall equilibrium in pressure, the increased interface with the air during a transfer can speed up the rate that the proportion of gases in the beer start to match those in the air.  And with that being nearly 21% oxygen and only 0.04% CO2, that's bad news.  I've learnt something today, thanks.

[klickitat jim]

Yup, I think.

Some will say that if the headspace were even 99% CO2 and only 1% O2, you would still get oxidation