To answer this question we need to decompose the question into two parts, first, why global carbon levels are the same, this has only one explanation, and this is because the amount of carbon on earth is constant, that is to say, the carbon on earth follows a cycle, can be in different forms but there are no new sources, there is no new carbon income into the planet, therefore, it does not matter how carbon is distributed on earth the quantities will remain the same.
Now for the atmospheric levels that is a whole new story and a very controversial topic, as global warming and carbon dioxide excess in the atmosphere increases some argue that is due to a natural phenomenon meanwhile others attribute it to human activity, so let's explore this two postures. In past eras there is a record of an increase in carbon dioxide due to intense volcanic activity, in the actual time it is said that this is due to natural activity and we are relieving a natural cycle due to volcanic activity, natural greenhouse effect, natural carbon dioxide as a subproduct of respiration and organic matter decomposition. On the other hand, there is the posture of human activity and the increase is given mainly by an excess of burning oil and subproducts (industrial processes, car driving among others), also intense clearing leading to massive deforestation decreases photosynthetic rate on earth, also water contamination leads to the loss of other photosynthetic organisms such as cyanobacteria and algae another important sink of carbon. Finally, there is a third explanation that what we are actually seeing is a combination of both processes natural and human, and together they make synergy, this posture has been recently gaining attention as there is proof of both causes, nonetheless, it is our responsibility to reduce our human impact.