WordPress 3.8 introduces new admin design that uses Open Sans as its font. Because of technical difficulties, it was decided that Open Sans is used by using Google Fonts. What this means is that on each WordPress admin page there is a stylesheet from Google Fonts included. See make/core thread and Trac tickets #26072 and #26063 for more details.
If for any reason you don’t want to use this setup, I made a plugin that disables loading from Google Fonts so visitor’s browser will fallback to its preferred sans serif font.