RDTSC is directly influenced by frequency scaling. So while monotonic, its clock interval is neither constant, nor deterministic on modern systems.

Here's a small online visualization of RDTSC average and standard deviation I just hacked together: https://gist.github.com/datenwolf/151486f6d73c9b25ac701bdbde...

On a system with frequency scaling you can see that under higher load the difference between RDTSC in subsequent iterations of a tight loop that does nothing else than reading that register will drop. Here's how it looks on the system I'm currently using: https://www.youtube.com/watch?v=FKKjSJ1JZ78