Essential qualification vs Higher qualification

Dear Experts, I am a little confused about recruitment rules. On 2015 there was an advertisement published by State public service commission seeking application for 10 post of Motor Vehicle Inspector. Essential Qualification asked: 1. 3 years Diploma in Automobile/ Mechanical Engineering 2. Should have knowledge in motor vehicle act, 1989 3. passed madhyamik examination 4. Should know bengali Desirable : holder of a Driving licence of all types of vehicle. But when the final result out we noticed that there are 4 candidates out there who are not a Diploma Engineer, but they are degree engineers. In that case, it is true that Degree is higher qualification than Diploma, but can the recruiting authority appoint those candidate who does not have diploma where it is clearly mentioned that Diploma is essential qualification for that post. N.B. In the Recruitment Rules, there it is also clearly mentioned that for that post diploma is essential. I hope you Experts understand my confusion and requesting your valuable guidance regarding that matter. What is the law of recruitment?