Mysteries of the Universe, Consumer Edition

Why aren't sizes for kids' clothing all the same? I understand the impulse behind vanity sizing for adults, and why that leads to different stores have different senses of what a size 6 might be. But this is craziness for toddlers. Why should Gymboree be bigger than Old Navy? Why aren't all 3T pajamas the same? I have enough jammed into my brain without trying to keep track of which brand's pants will fall off my son, and which will give only a month's worth of wear.