That's V-sync. It matches to the native frequency of your monitor - typically 60 if possible. If your GPU can't stay above 60, V-sync has to drop down to 30 to prevent tearing. Turn V-sync off if you don't want this feature.
V-sync prevents tearing (basically an artifact generated by moving while a frame is being rendered, causing the top half to be rendered a split second earlier than the bottom half, which renders a different position because you moved). The benefit is that at 60 Hz, its good - no tearing and you get as much information as your monitor can handle - 60 Hz for most, some can do 120Hz.
The drawback is that if you can't maintain above 60, it will drop you down to 30 so that it forces 2 monitor refreshes per frame rendered. For some, the no tearing is worth it. For others, the lost frames are too precious.
If you're unhappy about how it looks at 30, turn V-sync off and see how it looks.