It is implausible to me that there were no opinions pleasing to liberals that contained errors. Given the makeup of the court, it's possible that there just aren't very many liberal rulings, and none made it into the sample due to random chance. It's also possible that the liberal opinions were not examined and that the "moral force" of ProPublica's journalism is pushing to the left while trying to hide the bias from their reader.
There was a seventh error mentioned that was not covered in the article, and they mentioned that it would be the subject of a future article. Does anybody care to guess what direction the ruling will lean in that case?
Note: I'm making two comments on this thread since this is a completely separate point.