My Basic Pay fixed is not same as my 15 other colleagues recruited through same advertisement.

Dear Sir, I required a legal advise In ref to Pay revision of PSU (Cut off date: 01-01-17) How is it justified to have a different basic pay for 2 candidate selected through same advertisement No. But one get their offer & joined before 01-01-17 but another get their offer after 01-01-17? In ref to advertisement dated [deleted], written examination held on dated: [deleted] and subsequent personal interview held on dated [deleted]. Out of these total 17 candidate selected, 15 of them get their offer & joined before [deleted] & 02 including me get their offer & joined after cut off date of pay revision/fitment benefit i.e. after [deleted]. Now as per circular, employee joined before [deleted] is eligible for fitment benefit of 15% & employee joined after [deleted] is not eligible for such benefit. Which result in After addition of fitment benefit, the difference in revised basic pay for 15 of my colleagues and me so arrived is around 4.7%, which put me almost 1.5 year back (Considering annual increment @ 3%/Year) with my 15 colleagues having same qualification, same advertisement no., same date of written test, same date of personal interview and appointed on same terms & conditions. Your valuable advice is required on mentioned matter.