Everything we see, do and understand exists in a context window of an individual. We have a shared language, with which we can inexpertly communicate shared concepts. That language is terrible at communicating certain concepts, so we've invented things like math and counting to try to become more precise. It doesn't make those things "true" universally. It makes them consistent within a certain context.
How far it it from Dallas to Houston? On a paper map, it might be a few inches. True, within that context. Or you might get an answer for road miles. Or as the crow flies. In miles? Kilometers? It's only fairly recently (in human history) that we've even had somewhat consistent units of measure. And that whole conversation presupposes an enormous amount of culture knowledge and context - would that question mean anything to a native tribesman in Africa without an enormous amount of inculturation? Are their facts the same?
I'm not trying to make a "nothing is true, we can't know anything" kind of argument, that's lazy thinking.
I'm making an argument for maintaining skepticism in everything, even (especially?) things that you know for sure.