Maybe you lost your Wi-Fi signal for a moment? Or there is an app that is bypassing the Wi-Fi (WhatsApp has an option to switch to mobile network if the Wi-Fi is not good enough). You can track which app used your mobile network in your settings.
The 100 MB could be because the cellphone has background applications that use data. Even without knowing it, it is possible that you are using data when not on Wi-Fi. If you don't want to use your data outside your Wi-Fi, you can disable mobile data...
