based on the fact that I have raised two garters up from babies and bred them for the first time, and seeing such a wide variety of color and pattern in the clutch, I would say it's a pretty normal thing for them to have differences even with the same parents.
but hey, my mom and dad had three chilren, and we sure as heck don't look the same
i think that sometimes we as humans sometimes forget that animals even in the same species are quite capable of producing offspring of varied looks just as we are. It's like the deer that come to our backyard at the feeder. There is a select group of 9 doe that always come. To one person... they all look exactly the same. but to my family.. every one of them is very different looking and is recognizable in the group.