Most of the time when people talk about something being anachronistic, they're taking the context of the real world. But really, it's about context. If something makes sense within the context of your setting, it can hardly be anachronistic even if it doesn't match real-world expectations. Of course, this leads to the idea that settings themselves can't be anachronistic, which I don't think is quite true.
I was reading a scifi series a little while ago where the major alien species operated under feudalism. Feudalism, in a starfaring species. This is, I think, a setting anachronism. There's no reason for them to be feudal; they're rubber-mask humans. There's no excuses offered for something that should be impossible on the socioeconomic scale of a starfaring race of the same basic psyche as humans. The basic assumption of the setting had this completely out-of-place element, and it really destroyed the immersion I had.
In a different book, a fantasy with an expy of Edo Japan, the author kept using the word 'busted.' He had a busted lip, she had a busted eardrum. Some of it was verbal, some of it was narrative. In a setting where nobles (which these people are) are very formal, that sort of idiomatic English is horribly out of place. Again, it destroyed immersion.
In contrast, an in-universe anachronism is usually fascinating...and usually plot-centric. I'd love to write about a misplaced piece of high technology, or the remains of a Mayan railroad. It's just when they creep into the framing that they start destroying the integrity of your story.