So upon further testing, the miracle/numina/regen tissue seems *like* it scales with the level of the mob hit relative to your level (including exemplar values).
At level 50 +1 Incarnate shift, hitting a L 54 Nemesis minion with Dark Regeneration - slotted with Numina's and Miracle procs - yielded the first picture (Numina 0.11%/sec and Miracle 0.16%/sec)
At level 50 +1 Incarnate shift, hitting a L 20 Circle of Thorns minion (Archer) with the same power (Dark Regeneration) and procs, yielded the second picture (Numina 0.70%/sec and Miracle 1.05%/sec)
And lastly, when Miracle is slotted in health as per the usual, it grants 0.25%/sec