usage of interrupt for the same interrupt vertor number

hi
when we write codes like this:
void f1(void) interrupt 1
{ }
void f2(void) interrupt 1
{ }
There is a link problem because the different functions use the same interrupt vector number.
But if we write like this:
void f1(void) interrupt 0
{ }
void f2(void) interrupt 0
{ }
i.e. when several functions using the same interrupt vector 0, the code is ok. I find this in cypress's code sample for their autovector tricks.

would somebody explain this why?

thanks:)