"text scaling factor" is affected by system settings "scale for menu and title bars", and duplicates the text scaling.

[Lu Xu]

When Unity Tweal Tool is opened, the "text scaling factor" value is set to its initial value times value of "Scaling for menu and title bars" in system settings. And the text size will change again.

e.g.
If I set the "Scaling for menu and title bars" to 0.875 then open the Tweak Tool(I didn't change the setting in it before), the "text scaling factor" will be almost the same value, 0.88. After I click on the "text scaling factor" and click somewhere else(I think that's when the value change is detected), the text on screen will be smaller according to this value. In short, I think the text scaling is at last 0.88 * 0.875 in this case. (other things like button size is still 0.875)