It took me a while to figure out what was happening.
Mathematica 11 uses different fonts on Windows, OS X and Linux. They are Consolas, Source Code Pro and Bitstream Vera Sans Mono, respectively.
Some fonts use different descender styles for the Italic and Roman versions. Source Code Pro doesn't, so people on OS X cannot see this behaviour. But Consolas does.
The difference you are seeing is simply the difference between the Italic and Roman versions of the same font.
Mathematica uses Italics for pattern names and function arguments.
What you see is entirely normal and there is no problem with either your Mathematica installation or the fonts installed on your system. If it bothers you, you can change the default code font like this.