i might have to say that the plabt in the light might have been taking in to much light and it caused it to start degenrating due to the fact that to mcuh light can kill.but on the other hand the ne in the dark might have grown more perhaps it was in a dark moist spot where it the water you gave it was stuck in the enclosed room and keep the water in the plant and never died out... but idk just guessing
in the first few days of germination, and a plant's development, it requires a cool, dark place. during this period of time, it is growing off of the nutrients in the water, the soil, and what is in the seed.
during these first few days, a lot of light will do it no good, but once the nutrients contained in the seed are used up, it needs light
keeping it under light before even germination could be fatal to the plant, and it will stunt it's growth