There is another thread for this about provisioning writs varying between rank 6 and 7. They patched this to "fix" it by allowing both rank rewards to grant all the same rare items and master writs.

It's still not a very good drop chance and they do not award many master writs sadly despite it taking a very long grind to get the recipes they call for often. You can get a master writ calling for a purple recipe that drops from writ rewards and it rewards 2 writ vouchers while another master writ calls for one of the orzorga recipes from after the main quest of wrothgar for the same amount of vouchers. Definitely not consistent with other crafting skills.