You'll probably need to draw this one out to follow the example, sorry. Say the leader of term 2 got an entry partially replicated, then lost its leadership, then the leader of term 3 became leader and purged that entry from everyone's logs and replaced it with some other entry (which was also not committed). Now the server that was leader of term 2 becomes leader of term 4. It still believes that its entry was replicated to all those servers, even though they actually have a different entry now.
So basically, from one leadership to another, it's best to completely forget about what you think other servers are storing.
You'll probably need to draw this one out to follow the example, sorry. Say the leader of term 2 got an entry partially replicated, then lost its leadership, then the leader of term 3 became leader and purged that entry from everyone's logs and replaced it with some other entry (which was also not committed). Now the server that was leader of term 2 becomes leader of term 4. It still believes that its entry was replicated to all those servers, even though they actually have a different entry now.
So basically, from one leadership to another, it's best to completely forget about what you think other servers are storing.