Easy. The BBC was thrown out of Zimbabwe by Mugabe's government. Anyone who is publicly linked to the BBC is denied access to Zimbabwe. So the BBC uses other sources of information from inside Zimbabwe, including people who aren't officially BBC journalists or even journalists.
Clearly they don't want Mugabe's government to find out who these people are so they remain anonymous. The "official" BBC journalist collates this information, cross checks it, verifies it and reports on it from the safety of Johannesburg.