You're the OP, right?
The bias comes from the fact that you are emotionally invested with her. You have goals of keeping/improving a romantic relationship with your gf, and those goals at times may conflict with her parents' goals of...whatever they may be.
Also, you are often not there to hear the discussion, and your gf vents to you later. People who are venting leave off information that lowers the validity of their side of the argument. Humans adjust their own memories to make themselves look better too. This makes it very difficult for the 'outsider' to get a proper view of the personality of the other party. Really, it's best for nobody to vent to 'outsiders' about their family members, because they can really mess up relationships that way. But nobody lives like that very well.
I say biased because a) humans gonna human, and b) your desires for your gf to be a grownup are likely in conflict with the parents' desires not to lose their little girl. Not because you have any inherent inability to empathise or aren't a decent person.