I guess, because there is no difference in the characterization factors for N20 fossil und N20 non-fossil. So every process can simply emits "Dinitrogen monoxide" elementary flows (as this is the name for example in ecoinvent), independently of the naming for fossil or non-fossil origin.
In IPCC 2021 only a difference in characterization factors for "Methane, fossil" and "Methane, non-fossil" is made, since the factor for "Methane, non-fossil" is slightly lower due to the uptake of "Carbon dioxide, non-fossil" (in cases where "Carbon dioxide, non-fossil" uptake and emission is explicitly tracked as in EN15804 for example). And if there is no difference in characterization factors for "Dinitrogen monoxide, fossil" and "Dinitrogen monoxide, non-fossil" there is no need to differentiate it and it can simply accounted as "Dinitrogen monoxide".
See the original IPCC 2021 data here: https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_Chapter07.pdf
If you want to make the distinction between "Dinitrogen monoxide, fossil" and "Dinitrogen monoxide, non-fossil" in your inventories for yourself, you can also just add them as new elementary flows with the same characterization factor to your impact categories (a factor of 273 in GWP100 as an example).
