There is an extra scanline at the top of the NTSC signal that tells the CRT TV whether or not the output is interlaced or progressive. For a 60 fps signal, this extra scanline is filled once per 2 frames, such that 30 times per second, 525 scanlines are drawn: 262 for an A frame, 262 for a B frame, and 1 for switching between A & B.
The NES & SNES use a progressive signal, which effectively cuts the resolution in half, but boosts the frame rate up to true 60 fps, as opposed to interlaced, which is effectively 30 fps by requiring 2 passes to fully draw the "frame."
The mechanism for forcing progressive mode is to eliminate the 525th scanline. This tells the TV to continuously draw A frames only, and never switch to a B frame.
This doesn't account for the full calculation, but it's the general idea behind the faster frame rate as displayed on the tv.
As far as the TV is concerned, nothing is being bent out of the ordinary, because each individual scanline is still being drawn at the same rate as always.
With the full calculation, which I don't have in front of me, the EXACT frame rate for NTSC NES & SNES is a rational number expressed by the following fraction:
39,375,000 รท 655,171 = 60.09881389744...
If you use a high precision online calculator and plug in this fraction, you will see that it is a repeating decimal. (The first several hundred decimal places repeat en masse, such that you will see "9881389744" again way down the line.