The behaviour you describe is what would be expected.
Adaptive V-sync
Using this option, v-sync will only be enabled when you are maintaining 60fps. As soon as your system cannot sustain precisely 60fps, sync is disabled and you will probably experience tearing of the image. However the frame rate is free to change to any value (it may only drop to 59, for example) without consequence. If/when you return to 60fps rate, sync is re-enabled. This should make a pretty consistent and responsive experience for most players.
Adaptive (halve refresh rate) is exactly what is sounds like. The same as Adaptive, only the frame rate needed to maintain v-sync is 30 instead of 60. It will therefore cap you to 30fps.
Standard V-sync
Adaptive differs from standard v-sync. Standard v-sync will cap you to 30fps in an instant if you cannot maintain 60fps. If you can't maintain 30, it will drop you to 20, then 10 and so on. Standard v-sync is what you were forcing when you set v-sync on in the nvidia control panel and you were probably just dropping a frame or two here and there. The result is often an inconsistent, jarring experience as it caps you to 30, then 60, then 30 again and again.
It's worth mentioning again that the v-sync option selectable in the game itself is Adaptive.