[Grammar] since-clause tense

[hotapplepie]

Hello!
Could anyone help me with a question of tenses?
"Since" is used with two different kinds of tenses---simple past and present perfect.
Here is some examples I want to compare with:
1.He hasn't seen his brother since he has left his country.(since + present perfect)
2.He has lived here since he graduated from college.(since + simple past)
My question focuses on the tense of the since clause.
I was told the key of the tense of since clause is whether the action or status in the clause continues to the moment of speaking.
But recently I saw an example in Oxford dictionary:
"It's twenty years since I've seen her."
I am wondering why it doesn't use saw instead of have seen.
I would think "see someone" isn't an action that continues to now, rather an action at a specific time in the past. So why does here in this sentence use present perfect tense?
Thanks.

 

[5jj]

1.He hasn't seen his brother since he has left his country.(since + present perfect)

the first is not correct. It should be 'since he left'. 'His country' would normally be more natural as 'the country'.

"It's twenty years since I've seen her."
I am wondering why it doesn't use saw instead of have seen.
I would think "see someone" isn't an action that continues to now, rather an action at a specific time in the past. So why does here in this sentence use present perfect tense?

'Saw' would be fine. The present perfect, however, is sometimes heard in this type of implied negative construction. The speaker has in mind the thought 'I haven't seen her for twenty years'.