Data can be consumed by programs running on your devices (Desktops / Laptops / Tablets / Smartphones / Media Centers) that you aren't even aware of. This can include automatic system updates and in some cases even malware (viruses etc).
Another common cause is young family members who might be downloading large files such as movies and music without telling their caregivers in advance.
Also, with the advent of new video on demand services such as Netflix, it is common for data usage to be much higher than it once was. If you use any services such as this it may be something to factor in.
If you find that you are consistently going over your data limit you are welcome to upgrade to a higher plan to better suit your requirements.
You can review your usage history by logging into the account portal here: https://account.lightwire.co.nz/