Stepping back home (in our case, moving back to Australia after 2+ years living in London) is much the same - you have changed, and home has changed, but you don't expect either to be different.
I think a big part is expectations. When we left, we were expecting something different, unusual, full of surprises (how bad is London coffee?) and annoyances. But returning 'home' you have no such expectations - you just 'expect' to slot back in.
But you've travelled the world, learned a different culture (although I don't profess that in our case it was enormously different), moved on with your life in some way ... and so too have all those people who stayed behind, and indeed the city/town/country itself. So you're surprised, and you were expecting it to be effortless. Shock!
(And it is the weird little things. For the first year I was always one block off with my directions. But thankfully the coffee was as good as I remembered it.)