Redstone Torch BlockPower() returned values are inconsistent.

If torch is powered (through the block it's attached to), its BlockPower() value is 15.

If torch is unpowered, its BlockPower() value is 0,

unless

it is connected to (and therefore powering) at least two more pieces of redstone wire, in which case its BlockPower() value is 15.

Now, i've been scratching my head, trying to work out if this is intended behaviour but I can't see how it would be.  See attached gif for clarification.