0 light level on bottom level slabs

In areas where surface is a slab located in bottom of the 2 block positions, the light level is reported as 0 even in direct sunlight or under torch or other light placed directly above the slab {i.e. on wall beside the slab ). This occurs inside or outside and even on the low-slope slab stairs where all the low placed slabs report 0 level and the high placed slabs report the correct level. Condition exists inside and out and in single and multiplayer.

Don't know if this is a real problem,as it seems to violate spawning rules, but I have had mobs spawning randomly in my base and can find no other location where they might come from.