The phrase "since I've seen you" has connotations of meeting and discussion, whereas "since I saw you" implies that I merely saw you in the distance and it refers to a specific place.
For example: "It's been a long time since I saw you at the cricket ground".
This sentence infers NO meeting or discussion. The person may have been playing cricket and I saw him on the field.
And they always have an explanation why they avoid the grammar rules.
You Koronas should have been a lawyer, not a writer only!
Anyway, thank you for the explanation!
Never have I ever heard it before!!
I asked myself many times why some people speak that way. And I saw it in a book as well.
But the writer of the book didn't give an explanation.
I stress that I am not a teacher. However, I have been using English for nearly 60 years, so I can think to myself: "when would I say that?" Usually I can answer myself and give an explanation. Occasionally I get it wrong and someone else provides the correct version.
I don't know any rules. I just know what sounds correct. :)
And now I must go to bed.
(1) I found this in Mr. Michael Swan's popular book:
Sometimes a present perfect is used to refer to a finished point of time:
It is now a year since we have last discussed your future.
(More normal [says Mr. Swan]: since we last discussed ....)
(2) Kindly remember that Mr. Swan is writing from the point of view
of British English.
(3) Maybe (only "maybe") we can say that American English
favors "It has been a long time since I have seen you," but that "It
has been a long time since I saw you" would be OK, too.
(4) Other posters have already shared their great ideas. Let's
see what others think.
(1) If I understand that grammarian correctly, "It has been 4 years since I have
studied it" = I studied it 4 years ago. In other words, I stopped studying
it in 2006.
(2) If you want to say that you have been studying it from 2006 to today,
you should say:
It has been 4 years that I have studied it. (I am continuing to study it.)