So, this planet had been previously surveyed. Surely such an initial survey would reveal the technological level of the natives. Due to the unusual readings, Starfleet wanted Kirk to contact the inhabitants. Now, the landing party beamed down in uniform with visible display of phasers, tricorders and communicators.
Starfleet wanted the Enterprise crew to directly contact a pre-warp, technologically inferior culture.
The original survey was likely done from a distance without ever landing on the planet (else Vaal likely wouldn't have allowed them to leave). The strange reading likely led Starfleet/the Enterprise
to believe there was a more advanced culture than the one encountered by Kirk and Company.