no wonder - normally most inttrupts are redirected after boot. i presume u used a kind of debugger to have a look at IDT - it might well have happened that was the debugger who hooked INT 13. but if u trace the int handler code, u probably will find the reference to the original int handler (but, in principle, this has not to be the case - the functions of the original handler can be performed by another chunk of code).
regards