Typographic question: Width of non-breakable-space

Hi, everyone!

Why is the width of the "non-breakable-space" (e. g. ALT+0160) not the same as the width of the normal "space"? I could use the "narrow-non-breakable-space" but it's not available in everey OT-Font (e. g. Humanist777).

I can insert the &nnbsp from Arial in a text box with an other font - works fine. But if I work with styles and edit the style I lose the &nnbsp.

Any idea for me? Thanks for suggestions.