The Internet quota information displayed on the "check your quota" page may not be accurate. In cases where people have more than one registration for the same device (e.g. a device that is used in multiple buildings), the per-host graphs and the information displayed on that page may not always correctly aggregate all registrations. When this happens, that page only reflects the portion of total use associated with a single registration; other registrations for the same device are (incorrectly) ignored.
The parts of the quota system responsible for enforcement and generating warning email are, however, correctly aggregating this information. This leads to a situation where people are receiving an email warning (or an error informing them that their access has been suspended) whilst the "check your quota" page shows them to be under quota. This understandably is resulting in some confusion.
The per-host quota information that's used for enforcement and warning emails can be found at http://quotas.ru.ac.za/reporth.txt. Where the "check your quota" page and that report differ, the text report should be considered definitive.
The problem appears to have started when the server hosting the registration system and quota information was changed at the beginning of June, and is likely related to that change. As yet nobody has had a chance to look at what might be causing it.