Lightstone gives off 15 light, Torches give off 14.

Effectively a lightstone block is like having a block with torches on every side of it, sticking it 2 blocks behind loses an extra level of brightness, water acts as a -3, so your lightstone now gives off 10 (15 -2 - 3) lighting. Which is as much as a redstone torch, which also allows mobs to spawn. o_o