Political bias in history doesn't imply the historian is setting out to distort things. But you see different things according to your own personal history, even as an eye-witness.

For example it's quite to completely miss some aspect of an event - it's possible to look at a bunch of people in some historic meeting, and completely overlook the fact that they are all white, or all men. Entire peoples can be pushed to the margins in accounts of historical events. Gypsies in the Nazi death camps, black people in histories of the Boer Wars, Palestinians, native peoples...

It isn't intentional distortion, it's just not seeing things that you aren't focusing on. And we all do it, and sometimes realise years later.