Why data can be written at an offset memory location?

[sniper00]

        private void SetExtraObject<T>(T obj, bool weak) where T : class
        {
            var handle = GCHandle.Alloc(obj, weak ? GCHandleType.Weak : GCHandleType.Normal);
            IntPtr extraSpace = _luaState - IntPtr.Size;
            Marshal.WriteIntPtr(extraSpace, GCHandle.ToIntPtr(handle));
        }

[sniper00]

/*
** thread state + extra space
*/
typedef struct LX {
lu_byte extra_[LUA_EXTRASPACE];
lua_State l;
} LX;

/*
** Main thread combines a thread state and the global state
*/
typedef struct LG {
LX l;
global_State g;
} LG;