When reading about carbon offsetting you might read about carbon neutral air travel and climate neutral air travel. Although these concepts might look similar, they are different.
As one might know, a plane emits carbon. Emitting carbon at higher altitudes is expected to have higher carbon emissions, than when emitting on the ground. Since planes fly on higher altitudes, higher emissions factors need to be taken into consideration.
The exact factor is not certain yet, but the most renowned organizations, like the British government (DEFRA) and Dutch Milieucentraal.nl are advicing to take a multiplication factor of 1.9 into account.
That's why FlyGRN uses a factor of 1.9, to at least include some extra effects of carbon emissions at higher altitudes into account.
You can read more about it in the following article.