In MS Word, what dictates the "Original Size" of a bitmap picture?

I have a .docx containing a picture imported from PNG.

On a number of different computers, we see different information about the picture when we right-click => "Size and position…".

Some of these are Macs running Word for Mac 16.39. Others are Windows running recent versions of Office

The original PNG is 400 pixels wide. I unzipped the .docx and located the PNG within – it remains at 400 pixels wide. All the online PNG metadata tools I’ve tried report no pixels-per-metre metadata; OSX Preview reports 72 DPI, but I think it’s just reporting OSX’s own default.

• On everyone’s versions of Word, the dialog box does not report size in pixels
• On everyone’s versions of Word, "Absolute" width is reported as 10.58cm — this is the size we want, and corresponds to 96DPI, which is the pixel density we expect. (96DPI isn’t great print resolution, but that’s a different topic for another day).
• But the "Original size" varies, and since "Scale" is the ratio of "Absolute" to "Original" size, "Scale" also varies:
  • On some Macs, "Scale" is 75% and "Original" width is 14.11cm
  • On other Macs, "Scale" is 150% and "Original" width is 7.06cm
  • On all the Windows PCs we’ve tried, "Scale" is 100% and "Original" width is 10.58

In a sense none of this matters – the physical appearance of the printed document is the same in all cases, as is the apparent resolution of the image when zoomed in on screen.

… but it has led to some wasted time, as people debate the resolution and the scaling. So:

• What does Word mean by "Original size" and how is it calculated?