FreeBSD system call implementation

[Ravi Krishna]

Hi !
I am new to bsd internals and was looking at the implementation of syscall
in sys/i386/i386/trap.c
here its seems we get the handler for the syscall by 
if (p->p_sysent->sv_mask)
                code &= p->p_sysent->sv_mask;

        if (code >= p->p_sysent->sv_size)
                callp = &p->p_sysent->sv_table[0];
        else
                callp = &p->p_sysent->sv_table[code];

and call using 

if (error == 0) {
                td->td_retval[0] = 0;
                td->td_retval[1] = frame.tf_edx;

                STOPEVENT(p, S_SCE, narg);

                PTRACESTOP_SC(p, td, S_PT_SCE);

                error = (*callp->sy_call)(td, args);
        }
My question is why we store the p->p_sysent->sv_table
for each process. What is the reason for keeping this per process?
Are there some situations where two processes can have different system calls
available?

Regards,
Ravi