We know that the Earth's magnetic field undergoes regular reversals and during a reversal it is weaker and has some "holes". Instead of the Terrestrial N being delivered 3.8 Ga in the past, it could have been delivered during many short "windows" spanning all geologic time.

There is also the option of volatile-rich comets etc hitting the moon and influencing the N isotopes of near-surface material.

There may be other possibilities, too. Could enough N have streamed out from Venus over geologic time to do the same thing?

basically, I think there are many options and I would like to see these equally explored for plausibility before settling on a specific answer...