PyCharm is built on top of IntelliJ. IntelliJ has a default line length of 120 characters.
This is probably because you can’t fit a common Java name like: @annotated public static MyObjectFactoryFactory enterpriseObjectFactoryFactoryBuilderPattern { in a mere 80 character line. (I’m poking fun, but Java names do tend to be longer by convention).
The pep8 checker is configurable, so you can specify a better max line length – like 79 characters.
The error is misleading because the pep8 checker formats the text with something like "PEP8: line too long(... > %s characters)" % max_line_setting. So it’s using the pep8 checker, with a specific configuration, not claiming that pep8 specifies a 120 character line.