Philadelphia's city commissioners moved a step closer Wednesday to understanding why voters had to cast more than twice as many provisional ballots in the 2012 presidential election as in 2008, but the initial review raised as many questions as it answered.

Many of the problems occurred because of mistakes by poll workers and voters themselves, according to a preliminary report by Gregory Irving, the commissioners' acting voter registration administrator.

In 2012, 27,355 voters cast provisional ballots, up from 12,733 in 2008.

Provisional ballots are cast on paper when a poll worker cannot find a voter's name. Compared with machine votes, provisionals take longer to complete, are more complicated, and can be subject to challenge if the election is contested.

Anthony Clark, who co-chairs the three-member City Commissioners Board, said he believed the state's new voter-ID law contributed to the higher number of provisional votes.

Just weeks before Election Day, Commonwealth Court Judge Robert E. Simpson Jr. suspended implementation of a new state law requiring ID to vote. Many voters were concerned that they would mistakenly be asked for identification they didn't have, leading radio hosts and others to advise them to demand to vote by provisional ballot, Clark said.

"A lot of people were in the streets saying, 'Use provisionals, use provisionals,' " Clark said.

Irving said 19,670 provisional votes were ruled valid. Invalid votes included 4,240 from people not registered to vote at all or not in Pennsylvania, and 1,630 from voters who had been removed from registration lists. None of those votes was counted, Irving said.

Of the 19,670 valid provisional ballots, 5,263 were not in the poll books or on supplemental lists printed after the poll book deadline.

Some of the voters whose names were missing were registered to vote before they turned 18 but reached that age before Election Day. Irving said he was trying to determine why those names were not transferred to poll books or supplemental sheets.

Other names were missing because of an unspecified problem with a Department of State database.

Irving said that more than half of the people who voted by provisional ballot - a total of 14,407 - actually were listed in poll books or supplemental lists and would have been eligible to vote on machines.

But the voters either went to the wrong polling locations, or poll workers were unable to find their names. Irving was not sure why poll workers could not find them but suggested improved training was needed.