I need some help in understanding seedlings.

I have started all different kinds of seeds, and I was also taught and told never over water it is bad and can kill the plant. I have got many seeds to sprout into seedlings and are many weeks old. I am growing under an intense T5 4 foot 4 bulb floro light.

My question is I don't see noticeable growth unless I keep the soil wet, then I see the plant take off and grow like crazy but when I let it get only moist and let it drift into the middle of those soil moisture checkers they dont grow nearly as fast as keeping the soil WET. Why is that??