Then why can also the past perfect work after 'since'(also in the same case)?
You can't (if it's the same case). You use the appropriate tense for the context.
Here's a context:
The police are interviewing Peter about when he last saw John. At the time of Interview 1, Peter hasn't seen John since last year. There are two interviews.
Context 1. Peter hasn't seen John recently.
Interview 1. (Last week)
Police: "Have you seen John since you last spoke to him?"
Peter: "No, I haven't."
Interview 2. (Today)
Police: "During the last interview, you said you hadn't seen John since you last talked to him."
Peter: "That's true. I hadn't.
And I still haven't seen him since I last spoke to him."
Context 2. Peter saw and spoke to John yesterday (between Interviews 1 and 2)
Interview 1. (Last week)
Police: "Have you seen John since you last spoke to him?"
Peter: "No, I haven't." [Same as last time]
Interview 2. (Today)
Police: "During the last interview, you said you hadn't seen John since you last talked to him."
Peter: "That's true. I hadn't
seen him since I had talked to him last. But I spoke to him yesterday. Now, I have seen him since I had spoken to him last."
Seeing someone and talking to them is a bad example, since they both tend to occur at the same time. It would have made more of a contrast if you had chosen more independent actions.
The main point is that you
don't use two different tenses "in the same case." You use the tense appropriate to your meaning.
