Ballots can be pending for the following reasons:
Volume
By Pennsylvania law, ballots cannot begin to be processed until 7 a.m. on Election Day. The length of time it takes to complete ballot processing depends on the number of ballots received. It takes longer to count Absentee & Mail-in ballots due to the manual nature of the process, which is why results continue to update as eligible ballots are counted.
Identity verification
If a ballot is returned without the required identification information, Pennsylvania law provides up to seven days after the Election to submit the missing information. If the information is not received within the seven day period, the ballot will not be counted.
Awaiting further evaluation
Ballots with incomplete voter declarations, other potential defects or damage are segregated and examined to determine if they should be counted or rejected.
Sequester
Sometimes, ballots are set aside due to legal challenges by either of the political parties or candidates. Those ballots will continue to be set aside until the court issues a decision on how they should be handled.