Could it be that it's you who doesn't know which way is up? You said it yourself: you're new to the job and the company, so your judgment being off is more likely than everyone else being in the wrong.

See if you can tell the story from your colleague's perspective. Perhaps it might go something like this: "There's this one junior employee who seems to speak as if he knows everything, but in fact his fundamentals are lacking, so all I can do is set them straight as quickly as I can."