The left side was exposed to the plant light for the first few days. The other was only exposed to the white light. There is more or less the same mixture of plant types on each side so germination rates is not really a factor.
What is a plant light you ask? Usually sold as "Plant and Aquarium" bulbs, they are full spectrum bulbs meaning that they provide a broader spectrum of light than a traditional white fluorescent bulb and more closely mimic the spectrum of the sun. Plugged in, plant lights throw off a pinkish glow and they include some blue spectrum for leafy growth and the red spectrum for budding and flowering. For starting seeds, the lights that produce plenty of blue light are the most important.
They are usually available at most home improvement stores. We bought our original bulb at Home Depot for about $8 but online they can go for as much as $50 (don't pay that). Sadly our store seems to have stopped carrying the short length bulbs, so I guess we will be on the hunt for a new one at other stores in our area. In the meantime, I am rotating the seed tray every day to help even out the growth imbalance.
Here is what the package of our bulb looked like: