[solved] volume in Inspector vs. volume in Piano Roll Editor

In the Inspector, notice that the "velocity type" is set to "offset" - that means, this much higher or lower than whatever the default is as given by the dynamic marking in effect (which by default happens to be mf, not that you should rely on this). If you want to control the actual raw velocity in the Inspector, set the velocity type to "user" instead.

You wrote:
Then I open the same notes in Inspector and see that their volume there is 25!

The 'Velocity type' in the Inspector shows 'Offset', so the 'Velocity' value is taken as a percent offset.
In other words, you start from a (default) value of 80 and then increase it by 25%. So you get 1.25 x 80 = 100. This is the 'absolute' value shown in the PRE.

Then I try (still in Inspector) to set up the volume of the same notes to 100.

Okay, so you start from a value of 80 and increase it 100% (double the value). So you get 160 absolute value in the PRE.

Why is it so? Why not 100?

Try this:
In the Inspector, change the 'Velocity type' to 'User' and type a number into the 'Velocity' box, for example 100). Then the PRE (absolute) will show 100.

See:
https://musescore.org/en/handbook/3/inspector#note
Look for 'Velocity type')
and:
https://musescore.org/en/handbook/3/piano-roll-editor

Apparently continued in https://musescore.org/en/node/334087