Well, it does say that it heals you
to a specific number of HPs, not
by a specific number of HPs. Because of this, I think it's pretty evident that the spell doesn't care what health you have, because it will just take whatever number is in the Current HP variable and make it equal Full HP - 1. Sure, the wording could be a little better, but a better way to think about this is thusly: Miracle penalizes you for using it when you are at full HP, because it's stupid to do so.