If you'll look at the dates of the first descriptions of the various garter species / subspecies you'll see what they were faced with when naming / describing them and you'll see which were described to science first. Some subspecies names are named after the person who first described them to science. (such as pickering and fitchi) others are descriptive names, such as "concinnus". Eastern garters remained T. sirtalis sirtalis because they were named first as "sirtalis". So, when other sirtalis subspecies were named later, that first sirtalis became T. sirtalis sirtalis.

Makes sense doesn't it? It's abstract, but I can see how this happened. Just pay attention to the dates, and who first described them and you'll begin to see a pattern. Red sided are only called that because they were the first subspecies discovered with red on their sides. Not because they have it consistently or more so than other garters discovered later.