System: Windows 10, Indesign 17.1
Steps to reproduce:
1. Open 4 identical documents in A4 size, preferably single spreads
2. From the menu select to arrange them as "4-up vertical"
3. Press Ctrl-0 on all to fit the documents into the window
4. Compare the zoom value

In my case, on a full HD monitor, one document ended up having a zoom of 51.75% while the other three had 51%.