For the NTSC output that's reported as 50Hz, I imagine it's most likely because of the following FAQ:
Why is the refresh rate not reported correctly by utilities that use the XRandR X extension (e.g., the GNOME "Screen Resolution Preferences" panel, `xrandr -q`, etc)?
The XRandR X extension is not presently aware of multiple display devices on a single X screen; it only sees the MetaMode bounding box, which may contain one or more actual modes. This means that if multiple MetaModes have the same bounding box, XRandR will not be able to distinguish between them.
In order to support DynamicTwinView, the NVIDIA X driver must make each MetaMode appear to be unique to XRandR. Presently, the NVIDIA X driver accomplishes this by using the refresh rate as a unique identifier.
You can use `nvidia-settings -q RefreshRate` to query the actual refresh rate on each display device.
This behavior can be disabled by setting the X configuration option "DynamicTwinView" to FALSE.