VSync causes 40FPS -> 20FPS

I'm running a build on Windows created using 2017.4.18.

When choosing different VSync options, I observe the following:

VSync Off: 40 FPS
Every 2nd VBlank: 20 FPS
Every VBlank: 30 FPS

I don't understand why when using every 2nd VBlank would cause the FPS to drop to 20.
Shouldn't this still remain at 30 FPS?
The Unity doc says:
When VSync Count is set to Every Second V Blank it is capped at half the refresh rate (for example, 60hz is capped at 30 fps).
https://support.unity3d.com/hc/en-u...Target-frame-rate-or-V-Blank-not-set-properly


Is this expected to occur?