A tortoise shell will usually follow a species specific pattern....however, things such as light, diet, geographical location call all make a variation of the species pattern....
Not a stupid question at all! I've noticed with mine, that the dark grooves in between the scutes (light parts) get wider as the tortoises grow, leaving tiny light spots. Some tortoises, I've seen, are all one color. I'm not sure if this forms with age, or if the tort has to be born that way.
Cute little sully, too.