The character U+00A0 NO-BREAK SPACE (unbreakable space) is wider than
U+0020 SPACE (normal space) in fonts Bitstream Vera Sans and Bitstream
Vera Serif.

This is evidenced by running, say:

LC_ALL=C.UTF-8 pango-view --font="Bitstream Vera Serif 80" -q -t "$(perl -CSD -e 'print "x\x{020}x\x{0a0}x"')" -o test.png

which uses pango-view to create a test image displaying three 'x'
characters, the first two separated by a normal space and the latter
two by an unbreakable space.  The latter two are more widely spaced
than the former.

I believe this to be a mistake: unbreakable spaces are often used
(e.g., in Web pages) simply to indicate a space that should not allow
a line wrap: this should be visually indistinguishable to the user
from a normal space.  Having the two be of different width means that
many Web pages or other displayed text will appear irregularly spaced
(and that's sufficiently annoying that I noticed and tracked down the
culprit font).

I don't know who is responsible for this or how it can be fixed, so
I'm bug-reporting to Debian.