Formatting codes are stripped before calculating width, meaning that bold text returns a smaller width than it should.

Relevant code:
https://github.com/Chicken-Bones/CodeChickenLib/blob/master/src/codechicken/lib/gui/GuiDraw.java#L95-L99

See also MinecraftForge/MinecraftForge#1660

Must have been left from before bold text was added
Fixed in 16f0b50 (1.7.10-1.1.3.135)