In Philly, its not the percentage registered, its the percentage voting. What happens is that in certain precints there are literally no Republicans and the polls are not located in the most savory places. After the polls close, the RATS go inside and vote the entire roll. Thats why Though Philadelphia is a compact county it takes forevere for its results to come in. This is why the RATS complain so loudly at Republican efforts to monitor polling places. They claim it is racism, but the facts is it causes them to have to spend the time and money collecting the voters and bringing them to the polls.
Actually its both.
I don't remember the exact numbers (700,00 comes to mind), but the number of registered voters roughly equals the voting age population. It may even be a little more. The registration numbers suggest every adult in Philly is registered to vote. This is of course impossible, especially considering some of the adults are immigrants (legal or otherwise), felons, or simply not interested in the political process. Obviously many of the "voters" are phantoms who have long since died or moved.
If the voter rolls were more accurate, poll workers wouldn't be able to vote for the excess registrants. Nor would Dems be able to use walking around money to pay $10 per vote to people they bus/van to numerous polling places.
The key to reform in Philly, and other cities, is purging the voter rolls and starting over. But it won't happen. In some states the D's could never again win state wide elections without the big city fraud.