I think the general rule is that we (or at least I) tend to have a default assumption that others think like us at least on a fundamental level, and that they have fundamentally similar personalities. If we meet up with someone who has a different sort of mind / personality, we tend not to notice it unless it's really staring us in the face.

I may be unusual, but I'm exactly the reverse of this. My default assumption is that other people are very different from me. When someone thinks and feels like me, I'm always surprised, and it takes some time to convince me that this is stable. I suspect I may be kind of hyper-aware to individual differences, since I've paid a lot of attention to this in recent time