There are issues with the values loaded into the export presets.
I edited the frame rate on a custom export preset and saved it; the preset did not hold the value I entered. It appeared to be either picking up a value from another preset, or calculating a frame rate from the resolution entered. I have also had it load incorrect H and V resolutions, and it sometimes incorrectly loads Progressive instead of Interlaced.
Note: for interlaced video the frame rate is 1/2 the field rate. The frame rate is typically 29.97 frames per second for “NTSC”; the field rate (alternate scan lines) is 59.94 fields per second.
I can test this more exhaustively if needed.