2 different customers connected to same ISP have same Private AS Numbers

How is this possible?
A sample configuration will be good.

router 1 == internet == router 2

router 1 has as 65000 with customer A
router 1 has as 65000 with customer B

router 2 also has as 65000 with customer A
router 2 also has as 65000 with customer B

How is this possible? How did the ISP separate the 2 customer routes?