NTSC vs PAL

59.94 vs 60

In most cases "60" is techno-shorthand for 59.94, but not always. In fact, 59.94 is 99.9 percent of 60.

Progressive standards are usually described by their frame rates, such as 23.976, 24.000, 29.970, 30.000, 59.940 or 60.000p.
Interlaced standards, on the other hand, are typically based on field rates such as 60i.

Some video editors might say that 59.94 is a drop-frame version of 60fps, and in some ways they may be correct, even though the time code itself can be drop-frame (DF) or non-drop-frame (NDF).
Knowledgeable engineers know that 59.94 fields per second is a legacy of the NTSC color system, because prior to 1954, black-and-white video was locked to the 60Hz frequency of standard AC current in the United States.
The 29.97Hz frame scan rate was designed to make color TV backward compatible with black-and-white TV when color was introduced.
A frequency offset of 0.03Hz was introduced to make space in time for the color sub-carrier.

29.97Hz frame scan rate 是为了兼容黑白电视。

与PAL制相比，N制的帧率基本相当于每1001帧丢1帧。

Different parts of the world use different timecode frame rates. The most common ones are:

24 frame/sec (film, ATSC, 2k, 4k, 6k)
25 frame/sec (PAL, used in Europe, Uruguay, Argentina, Australia), SECAM, DVB, ATSC)
29.97 (30 ÷ 1.001) frame/sec (NTSC American System (US, Canada, Mexico, Colombia, etc.), ATSC, PAL-M (Brazil))
30 frame/sec (ATSC)

Why the different frame rates around the world?

Because originally television was only live and not recorded, so the only way to ensure sync between studio cameras and home TV sets was to sync the signal to electrical mains.
Mains were 60Hz (30fps) in the US and 50Hz (25fps) in Europe.
But television was originally in black and white, so when color came along, SMPTE engineers wanted to keep the color signal backward compatible with black and white TVs.
To do that, they had to “slide” a color signal between the existing black and white signal and slightly alter the frame rate from 30fps to 30/1.001 = 29.970 fps to avoid artefacts, thereby creating the NTSC color standard.

moved the two signals out of phase

So the frame rate of television was actually exactly 30 frames per second at one point in time. However that all changed when color television was introduced.
When a signal for color information was added to the television transmission there was a big problem.
The color carrier signal was phasing with with the sound carrier signal because they were very close in the spectrum.
This made the picture look un-watchable.
The quick fix they came up with was to reduce the framerate by .03fps which moved the two signals out of phase.

The horizontal line rate was reduced

29.97 frames per second is a legacy of the transition of the NTSC standard for broadcast television from black and white to color.
In December 1953, the FCC made the transition to color television, but required full black and white backwards compatibility.

To accomplish this, they encoded the color information on a subcarrier frequency.
The frequency was chosen so that the color (chrominance) information interleaved with the black and white (luminance) information when the receiver processed the modulation components.
This allowed black and white TVs to filter out the chrominance information from the luminance signal.

The frequency divider circuits at the time were limited, and so the subcarrier frequency had to be set at 3.58 MHz.
This necessitated a slight change in the luminance signal, in order for the carrier frequency to correspond the subcarrier frequency.
The horizontal line rate was reduced from 15,734 lines per second to 15,730 lines per second, and the frame rate was reduced from 30 to 29.97 frames per second.
The variance was small enough that black and white TVs could still tolerate the broadcast signal, while allowing color TVs to display color.

The NTSC video standard survived until the FCC mandated the switch to digital broadcast television recently.

References:

https://www.tvtechnology.com/news/whats-the-difference-between-5994fps-and-60fps
https://en.wikipedia.org/wiki/Talk%3AFrame_rate#Accuracy
https://blog.frame.io/2017/07/17/timecode-and-frame-rates/
https://en.wikipedia.org/wiki/NTSC
http://theautomaticfilmmaker.com/blog/2009/2/23/about-frame-rates-or-why-2997.html
https://zhuanlan.zhihu.com/p/32662929
https://www.quora.com/What-is-the-difference-between-29-97FPS-and-30FPS