[UberHammer]

The only reason cancelled trips don't show on your log is because the unique trip ID of the request got picked up by another driver and ends up on their log because they were the last driver that had it. When you cancel and the request doesn't get picked up by another driver the unique trip ID ends up on your log because you were the last driver that had it.

 

[UberHammer]

I hope this is still the only case when it shows, but it is doubtful (today) given the amount of pings showing in history.

It would depend on how they setup the SQL database, but it's very unlikely that a unique trip ID could be tied to two drivers simultaneously.
If it's on your trip log, it's because it never got picked up by another driver after you canceled.