No problem, you're welcome.
Many thanks @tetukowski for sharing this.
One or two may say this is a bug, but anyhow, there is room for improvement in sense of handling metric and imperial units and their conversion between drawings.
Concerning the
Info tool
Distance Point to Point, the output doesn't show a unit.
It is meant to measure nameless drawing units and it is dedicated to the draftsman to interpret them and set preferences correct.
Changing Current Drawing Preferences does not perform any conversion to the drawing.
So one unit will be one unit before and after, but it will be interpreted differently.
Doing automatic conversion could be good or wrong, it will have proponents and opponents.
I can imagine a warning message, when application and drawing preferences don't match and possibly offer a conversion on opening or pasting.
At least a warning could point to the discrepancy and avoid confusion when things don't match.
This may also be an issue using blocks and libraries from different sources.
When you say, the source drawing was set to Inch and you measure correct length in mm, I assume something is fishy with that drawing.
At least, you are aware about the cause now and can fix it with
Modify -> Scale tool if needed.
investing less than half an hour into Search function can save hours or days of waiting for a solution