What causes the counter-intuitive presence of CO2 in the atmosphere during spring and summer months?
From Scripps:


  [The] latitudinal differences in fluctuation are the result of photosynthetic activity by plants. As plants begin to photosynthesize in the spring and summer, they consume CO2 from the atmosphere and eventually use it as a carbon source for growth and reproduction. This causes the decrease in CO2 levels that begins every year in May. Once winter arrives, plants save energy by decreasing photosynthesis. Without photosynthesis, the dominant process is the exhalation of CO2 by the total ecosystem, including bacteria, plants, and animals.  Two regions of Earth contain the majority of land plants: Northern Hemisphere continents and the tropics that encompass ... the vast rain forests of the Amazon basin. Near the equator, however, the seasonal variations in temperature are less pronounced. Seasonal swings in CO2 are therefore most pronounced in the Northern Hemisphere, where the seasonal changes in temperature result in very large differences in plant photosynthesis from summer to winter.


My interpretation of the slide-graphic confirms the idea that more CO2 exists earlier in the springtime, especially in the Northern Hemisphere.  There are darker reds (~403 ppb) over most of the northern latitudes in March/April



and more of an orange-yellow color in June (~400 ppb).



In general, I would caution you in putting too much meaning into graphics that NASA develops such as these.  These graphics are made by concatenating a couple weeks of satellite overpass data (generally one to two retrievals per pixel).  These are not an "average" like you might think.  This is because the swath of the instrument detection is very thin, thus requiring weeks to get "global coverage".  Furthermore, the OCO CO2 algorithms are still being refined, and will go through several more revisions before the data is "finalized".