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Your hunch is correct. AIP ENR 1.1 para 2.2.12 says that if you are in CTA, you should remain 500ft above the lower limit if CTA to provide an adequate vertical buffer with aircraft that may be operating in adjoining Class G airspace. See also AIP ENR 1.1 para 18.104.22.168.
This allows aircraft that are below the CTA steps to fly at the lower limit and still be considered OCTA.