Quoting Ted Sanders

Ted Sanders explains that LLM performance is multidimensional, but much of the variation is explained by model size: bigger models are generally more performant and reliable, cost more per token, and better recall esoteric facts. He notes OpenAI's models are ordered Luna < Terra < Sol < Astra, and that version numbers now signal release order, relationship to other releases, and magnitude of improvement—so 6.1 came after 5.6, may resemble other 6.1 models, and isn't a large enough leap to be a 7.