Pragmatically speaking, that suffix use to feminize the name. "Wati" or as you said "Ati".
For example "Karyawan" which is refer for males employer or general employer without any specific gender explanation, if you want to feminize it just change the "awan" to "wati" which is "Karyawati". As it also apply for "Mega-wati" which is the name of the former Indonesian president.
Also "sih" mostly use to feminize the name. Like Sriningsih, Sukaysih, and all of that. Like the name "Jacques" and "Jacqueline" both are identical, the "line" just use to feminize the name. Or "Paul" and "Pauline". Or in Arabic, "Jameel" with "Jameelah" both are identical in meaning, but one is for male while the other is for female.
So do Wahyu and Wahyuningsih, one is for male while other for female.