Why is 0.4 deg the preferred resolution for the extended (EXT) forecast?

I have come to know 0.4 deg as the most common resolution for the EXT forecast members. The stated resolution of “about 36km” is closer (at the equator) to 0.3 deg. Is the choice of 0.4 a historical artifact? Or is it that, in the subtropics, 0.4 deg is close enough to 36km?