The deaths occurred on Wednesday when a bomb went off at a site near a mosque where voters were being registered.
The US military said the blast killed six people - two employees of the United Nations Assistance Mission in Afghanistan (UNAMA) and four "local nationals".
But David Singh, spokesman for the United Nations in Kabul, and the governor of Ghazni both said they knew of only two fatalities.
"Preliminary information we have shows that an explosive device went off in a voter-registration site located in a mosque in Andar district," he said, referring to an area about 160km southwest of Kabul.
One of those killed was a member of the Joint Electoral Management Body (JEMB), a UN-government organisation coordinating preparations for a landmark presidential poll scheduled for 9 October and parliamentary elections in April.
Singh said two JEMB staff were seriously wounded and evacuated to the US military headquarters at Bagram, just north of Kabul, and four more JEMB staff were slightly hurt.
"Preliminary information we have shows that an explosive device went off in a voter registration site located in a mosque in Andar district"
UN spokesman in Kabul
Ghazni governor Haji Assad-Allah said two Afghan men were killed in an attack he blamed on remnants of the Taliban and its allies, who have described the planned elections as a US-orchestrated "drama".
It was the worst incident on Afghanistan's election preparations since 26 June, when three Afghan female poll workers were killed in the eastern city of Jalalabad.
A day earlier 16 passengers of a bus who were killed, were found carrying voter registration cards, according to provincial authorities.
Despite the violence, the US-appointed Afghan government said more than eight million out of an electorate of about 10 million have registered to take part in the elections.