Have you ever noticed that your own culture is invisible to you?
I think the most obvious example of this is accent. When you hear someone with a different accent speak, you notice their accent. It sounds weird and different to you and you recognize it as an accent. But when you speak, or you hear someone with the same accent as you, you don't notice it. It just sounds like they're talking normally and it doesn't even register that that's an accent too. But it's not hard to realize that you do have an accent, you just don't notice it.
Pretty much every cultural trait is the same. Consider for example social signalling. When someone from a different social group than you signals something, it looks like a waste of money. When you do it, you don't even realize that's what you're doing.
Your culture is the baseline against which all others are judged. Anything that differs from your culture is weird and unusual. Anything that's the same is just the way things are supposed to be.