Inset/Outset steps - distance is incorrect

Steps to reproduce:

1. Edit > Preferences > Behavior > Steps - Change "Inset/Outset by:" to 10 mm.
2. Draw a rectangle, W=100 mm, H=100 mm.
3. Path > Object to path
4. Path > Inset

What happened?

Rectangular path is W=24.675 mm, H=24.675 mm.

What should have happened?

Rectangular path should be W=80 mm, H=80 mm.

Notes:

1. Similar bug occurs when using Path > Outset.
2. Linked Offset is not a good workaround since it rounds off the corners (and AFAIK I can't specify the offset size in mm).

Inkscape Version and Operating System:

• Inkscape Version: Inkscape 0.92.4 (5da689c313, 2019-01-14)
• Operating System: Microsoft Windows 10 Pro
• Operating System version: 10.0.18362 Build 18362