It was a terrible oversite. One of the mods commented a while back that they know this is imbalanced. Both BLM and PLD were gimped compared to other classes. It's why they fixed the mirror issue in Levi X by allowing 1 mirror to upgrade both the shield and the weapon rather than forcing healers and tanks to obtain 2 mirrors for the full upgrade. I assume it's also why the high Allagan shield is iLevel 115 and drops in T8.

The ex primal shield for BLM should have been i95. The PLD shield from BCoB should have dropped in T4 and been i95. But keep in mind when twin was first downed all weapons were i90 only. They upped the iLevel later because there was little incentive for players to kill Twintania at all when the relic +1 (as it was called then) was so much easier to get.

Long story short, the game was released before it was ready and it's things like that which got overlooked as a result.