When Lu Ming initially began to light up the lanterns, they just seemed to be sources that could be controlled using fire energy. Since he followed the Keeper's advice, Lu Ming was able to light the lanterns quite easily. The energy expenditure was also on the minimal side because the training of 4 years within the Crimson lord world, improved Lu Ming's fire control capacity to a monstrous extent. One couldn't begin to imagine how efficient he was in power expenditure.
It was not until he lit the 10th lantern up that Lu Ming began to feel awkward. Something didn't seem normal about the set of lanterns. At first, he thought it was the vast difference in energy expenditure while lighting them, but soon enough a strange pattern seems to have presented itself.