How can Methane be less concentrated in the atmosphere but more harmful?

Like nitrous oxide, the concentration of methane in the atmosphere is measure in parts per billion (about 1850 ppb) whereas carbon dioxide is measured in parts per million (recently having crossed 400 ppm).  This means that carbon dioxide is in the range of (0.0004/0.000000185) = 2,000 times more concentrated.

On the other hand, if we compare a kilogram of concentrated carbon dioxide (GWP = 1) with an equivalent Kilogram of methane, we find the latter has a Global Warming Potential of 86 times carbon dioxide (on a 20-year basis), and 34 times (on a 100-year basis).

Although methane has a greater GWP, because its concentration is so much lower than that of carbon dioxide, the latter is overall a more important warming gas.