A: Could have + past participle means that something was possible in the past, or you had the ability to do something in the past, but that you didn't do it.
Example :
1. I could have stayed up late, but I decided to go to bed early.
2. They could have won the race, but they didn't try hard enough.

B: We use could have + past participle when we want to make a guess about something that happened in the past. In this case, we don't know if what we're saying is true or not true. We're just talking about our opinion of what maybe happened.
Example :
Why is John late?
1. He could have got stuck in traffic.
2. He could have overslept.