Unless there was more co2 in the air back then. Wikipedia .. CO2 today is 407 ppm (parts per million) ... Cmbrian period 500 million yrs ago was 7,000 ppm That to me is the answer. And the reason Oxygen was 35% was due to the tremendous amount of plant life giving off 02.

https://en.wikipedia.org/wiki/Carbon_dioxide_in_Earth%27s_atmosphere