My phone is Lenovo A6000+
Original charger and Stock OS.
Edit: it is a two months old phone with no other complaints.
The only reason I can think of why your charges faster on WIFI, with the information given, is you have data enabled. If data is enabled and WIFI is enabled your phone uses WIFI. However if data is enabled and WIFI disabled, your phone will use data, which on my phone uses 1.5 times more power than WIFI with a good connection. You can disable data by going to Settings>mobile data>off (on some phones it is under the "More..." settings).