In DejaVu sans mono, the degree celsius sign ℃ is too thin and not really sans serif. It is also too wide, which may be intentional, though. The URL points to an OpenOffice screenshot on Ubuntu Jaunty.
DejaVu Mono doesn't have the degree Celcius symbol. What you're seeing is glyph substition in process: since the font doesn't have the symbol, the renderer will look for it in another font to display it on your screen, so you wouldn't see a black box or so. Hence the style is completely different and the width doesn't match. By the way, it's not really recommended to use the degree Celcius symbol U+2103. It's best to type degree and C like °C.
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