Comment by adrian_b

That syscall man page is from glibc and provides information about how you can invoke syscalls through glibc.

It does not have any direct connection with the Linux kernel. Because the Linux kernel promises that the syscall interface is stable, in the normal situation when the kernel is newer or at least of the same age with glibc, all the syscalls that can be invoked through glibc should be supported by the kernel, but the kernel may support extra syscalls.

If you install a kernel that is older than glibc, which may happen in some embedded systems that are compatible only with some old kernels, then it may be that the kernel does not support all the syscalls from the glibc headers.