The focal length of unit-focusing lenses such as the 105/4 micro does not change when focusing. The focal length remains at 105mm whether at infinity, or 1:1 (with PN-11) or any other distance. The reason the angle of view gets narrower when focusing closer is that the optical center of the lens gets further from the image plane. At infinity (no extension) it is 105mm from the image plane. At 1:1 the distance increases to 210mm (105mm of the lens itself, plus 105mm extension). That's where the 210mm comes from. Since the lens is further away from the sensor, the angle of view is narrower.
The point I was making is that the term "focus breathing" is used to describe two different things (which is confusing!) For still photography, when a lens focal length changes drastically when focusing, it is known as focus breathing. It usually shows as a reduction in focal length when focusing close, which means the framing is wider and the magnification is less than expected. For example, compare the 28-300 zoom @300m at close range, compared to a 300mm prime.
Video photographers usually want the framing and subject size to stay constant when focusing. If the framing expands or shrinks that is also known as focus breathing. This effect is not usually of concern in still photography. If a unit-focusing lens is used for video, the framing will change as explained above. To counter this, the lens focal length needs to change to compensate. So to fix focus breathing (video) you need a lens with focus breathing (stills)
It would be useful if we used different terms for these effects since they are quite separate...