@Cannon Goose, two reasons for that:
1. RTSS can't read it's own frametime when the framerate is above/limited by the limiter. If you have a G-SYNC monitor and a built-in refresh rate meter, RTSS will also show slight framerate fluctuation (usually 1-2 frames).
2. RTSS sets a frametime target, while most in-game limiters set a framerate target. This is the reason why RTSS has steadier frametimes over in-game limiters, and the main reason in-game limiters have lower input lag than RTSS is because they can use those fluctuating (usually lower) frametimes to render and deliver frames faster.
If you have G-SYNC, unless the in-game limiter and/or engine are downright horrible and impose some sort of their own frame pacing, frametime fluctuations are smoothed out by G-SYNC's frametime compensation mechanism (the G-SYNC + V-SYNC ON combo that prevents tearing at all times).
I did some high speed camera input latency tests specifically for Overwatch in our forums, and the in-game limiter had much less input latency than RTSS and performed just as smoothly with G-SYNC.