Open device portal by typing in browser 192.168.8.1 or install HiLink app from Play store and see how much data was actually consumed. If that matches with Zong sent alert, then you actually consumed that.
Do not run youtube videos in 720p or 480p. 15 GB is not enough for that.
Do not repeatedly run speedtest. Every test will easily consume 100 - 300 MB.
Disable auto updates on phone and windows.