Can anyone answer a few questions about apm UI?

[wajika]

1. First I found a request, its status is http200, but it also contains an error message, I wonder why http200 also returns an error? And where can I find the details of this error?
   

   

2. I can understand that occurrences represent the number of times in a certain time range, so what is the previous period compared to the data generated?
   

   
   
   
   

3. Similar to the first question, the error tag appears but the error tab has no data
   

   
   
   
   
   
   
   

Thanks in advance for any reply