Since-clause

Since can be used as a conjunction of time, introducing its own clause. The tense in the since-clause can be perfect or past, depending on the meaning. Compare :

ex1) I've known her since we were at school together

I've known her since I've lived in this street

ex2) You've drunk about ten cups of tea since you arrived

You've drunk about ten cups of tea since you've been sitting here

ex3) We visit my parents every week since we bought the car

We visit my parents every week since we've had the car

from Practical English Usage by Michael Swan

Q. What is the meaning difference between the two sentences in each example ?