The general dosimeters available at Chernobyl could only read up to 3.6 Roentgen, so they took that value and said “hey, that’s not a too bad a level of radiation” without realizing (or putting in their due diligence) that the true reading was over 15,000. And the head operator and architect of the disaster (dyatlov) kept using 3.6 as the proof that everything is alright and just a minor setback even though the only higher range dosimeter that was available immediately fried itself