It'd be a bit weird to have the Sonnet numbering ahead of the Opus numbering. The Opus 4.5->4.6 change was a little more incremental (from my perspective at least, I haven't been paying attention to benchmark numbers), so I think the Opus numbering makes sense.
Maybe they're numbering the models based on internal architecture/codebase revisions and Sonnet 4.6 was trained using the 4.6 tooling, which didn't change enough to warrant 5?