If Employer doesn't mentioned about anything in advertisement but In admit card they add content

Sir, Initially NTPC LTD released a advertisement for recruitment of experienced engineer dated 06.08.2019 with eligibility criteria candidates should have 4 years degree and 3 years post qualification experience in thermal power plants. But after 6 months dated 23.03.2020 Admit card is released with imposed conditions 3 years of experience excluding training. I was employed to CREWPL (which is an Vendor ho PTPS Power) I joined with offer letter as GET and I worked there since 19.11.2015 to 07.09.2018 after I resigned from there company issues a experience letter on which it is clearly mentioned that I have been working as full capacity of AE since 19.11.2015 to 07.09.2018 and then in BEL since 11.02.2019 as Contract Engineer so total experience will around 3 years and 4 months which is more that experience Requirement in advertisement but after qualifying every stage of exam NTPC denied to appointment me- with reason that I have not eligible and not having complete experience. Kindly enlightening it with your legal knowledge and experience. What should be right decision. ?