I think you're mistaking their symbol for "a different font". Each one of those has a logotype and a symbol, which make up their visual identity (the word 'logo' is used interchangeably for those, or their combination). They are merely using the existing symbol as favicon - the stylized N/B/B. Google's symbol at the time was the uppercase G in the same font, so creating a new one for the favicon would be considered a rebranding.