Correct - one significant difference between a UNICOM and CAGRS is that
the UNICOM is not necessarily a dedicated service that will always be available when called; the nature of UNICOM is that it may be a secondary function to the commercial activities of the operator, e.g. refuelling, aircraft hire, pilot shop.
(CASA Summary of Responses - Regulatory Standards for Airspace - CASR Part 71).
A CAGRS on the other hand is a dedicated continuous service. So a UNICOM operator could indeed be away doing something (loading bags, refuelling) just when needed.
I recall a trial some years ago of one of the first UNICOMs here where I'm told the gent was a baggie for an airline. When the aircraft was inbound, he really didn't want to be manning the UNICOM but instead prepping the bags etc. etc. for quick turnaround outbound flight. If there was a competing airline there, would he have waited around for them too? I suspect not.
As I've quoted here before from the same document:
UNICOMs originated in the USA, however, CASA understands that the FAA does not include them in any risk mitigation hierarchy for airspace and traffic services;
and
The higher level services such as CA/GRS have certified, or licensed operators which provide a dedicated, continuous service provided by the aerodrome operator. Because of the higher level of competence that is required to obtain certification, CASA permits the assessment of traffic so that only relevant traffic is passed. In addition, the CA/GRS operators are qualified and authorised to provide meteorological observations including cloud and visibility assessments, and an approved source QNH which can be used for the purposes of lower instrument approach minima.
Those CA/GRS now in operation have been shown to enhance safety and have received a high level of pilot acceptance. CASA intends to retain the standards for CA/GRS service provision, as a cost effective service for use at high traffic density regional non-controlled aerodromes.