The phraseology means nothing significant.
According to good investigative practice a death is treated as possible murder until it is clear there is another explanation. This frequently requires a wait until after a post-mortem examination (hours) and sometimes associated toxicilogy results (days, or even weeks).
The media (being simplistic) always jump up and down saying "Is it being treated as murder? Is it being treated as murder?" and, also being congenitally incapable of distinguishing between "being treated as" and "is", usually run off and print headlines reading "Horrible Murder, Horrible Murder" which (a) distress families unnecessarily; (b) add unnecessarily to the communities fear of crime and (c) frequently cause actual problems for the investigation.
So the phrase "It is being treated as an unexplained death at this time" was invented.
But the police aren't led by the media in what they do at all. Oh no!