Weird Skylight value after reloading a map

I built a 100 by 100 platform (made of white concrete) at a high place where y = 100. There is nothing above it except for a roof made of glasses (block_id 20) with a torch (block_id 50, block data 5) placed on the top of each glass block. Everything looks right after I built it. However after I save & quit and reloaded, some weird shadows appeared as shown in the screen shot. The Skylight value of the blocks above the concrete blocks (with shadow), which were originally 15, became 14 or 13. I put a concrete somewhere in the shadow and everything went normal. But this did not fix the problem: I save & quit and reloaded, the shadows came back, and of course the skylight values became less than 15 again. Moreover, those "shadows", or say, blocks with incorrect skylight values seem to appear randomly.