custom FBA signin Page folder location

Hi 

web application custiom signin page URL I have given "/_layouts/15/GG.Login.Custom/Login.aspx" but when I physically check the location the  GG.Login.Custom sitting under Template/_layouts/16/ but when I give the location 16 its not working its only working when I give the location folder to /15/ is there any specific reason for the SharePoint considering only certain folder locations?

Thanks