I thought it meant that they were double redundant? I.E. speculation that there are two sets of generators and two sets of shields? It makes as good sense as any.
As for regular shields, I assume that they absorb energy up to their capacity (ie, shields at 30% would mean that the generators are up to 70% of their capacity), then they fail. As this capacity goes down, the ship is liable to more and more damage from enemy fire. Some energy will seep through when they are down to 30%, but not when they are at 100%. I think this is consistent with what we have seen.
Also, there are divided into parts (ventral, port, etc), so sometimes, when they say shields at 40%, it could mean that overall, they are at 40%, some might be lower, others higher, unless specifically indicated, such and such shield at such percentage.