Why if in the same place it says that you have unlimited VPN traffic?
The word "Unlimited" in advertising has little connection to the word "unlimited" as found in a dictionary.
In advertising, "unlimited" really means that there are non-disclosed limits that, upon being reached, mark the one reaching them as acting in an "abnormal" or "non-intended" fashion.
I am speculating based on my experiences with the VPN and my knowledge that we live in a world of limits, not a world of magical unicorns and free lunches.
Someone from Opera is free to come and offer a more plausible explanation of the observed behavior, of course...