I am on a 60gb per month plan, recently my data is being eaten, as a test last night I turned off my wifi device at 25.6 usage, this morning when I turned it on my data is 23.2.
It can take up to 24 hours for the data statistics to update on your My EE account (although usually it updates quite quickly). This could account for the data used before you turned off your WiFi device.
