This is a complex answer, so feel free to ask more questions.

Basically, one major difference between the images is that your noise sources in the image is vastly different. Because the noise is different and because its contribution is different with regard to exposure, the obtained images are quite different.

Think of your camera detector as a well that measures electrons. That well captures either light (photons) which is converted to "good" electrons or "bad" parasitic electrons that are caused by system noise (camera).

1. when shooting in long exposure in low light scenario the dominant noise source is usually the "dark current" of the detector. you can read about it more on wikipedia but its basically noise that is caused by the detector itself being active. dark current increases in a linear manner with the exposure. ergo, the total measured current in an exposure is I_dark_current*t_exposure.
2. With the room lit, the light signal is vastly greater than the dark current, your system dominant noise is a different noise source called shot noise. Its very interesting, and its root is related to quantum mechanics and if you have the time and curiosity, you can also read about it in wikipedia.

The main difference, with regard to your question, is that the shot noise is proportional to the square root of the input signal. so for example, if you increase your exposure by 2 and capture twice the "good" electrons, your noise would increase by the square root of 2. while in a dark room, increasing the signal by 2 would increase the noise by 2 as well! This means that for low light, once you are shot noise dominated, increasing the exposure would increase the signal more than the noise, which is desired if you want to capture the actual details in the scene.

This is a very important distinction and for this reason, a camera designer would always want his system to be shot noise limited when possible in order to obtain the best signal to noise ratio.

Hope this shed some light on things :)