Windows, like most other operating systems, doesn't have a stable user-kernel boundary. Instead the stable interface exists at the C level in libraries used by every process. This is no different from the situation in macOS (libSystem.dylib) or the various BSDs. Linux is the outlier because Linux is just a kernel; without an integrated C library, it by necessity has to keep the syscall numbers constant. Most other operating systems are free to change syscall numbers from release to release.