When was the last time you bathed?
No, JK on that...
There seems to be two types of dragon encounters in the game.
"Random" ones where the dragon spawns and attacks anything near it, and ones associated with a location (like an outdoor word wall.) (Not including named ones you encounter at specific points.)
What is likely happening is that you are seeing the ones associated with a location as they fly around. There is probably a word wall nearby that they are circling. They won't attack you unless you approach the wall.
For example, I see one quite frequently around Winterhold that simply will not land or attack. It circles 5 or 6 times and then flies off, probably back to the word wall it is associated with.
I hope that's clear enough. Someone familiar with the actual mechanics may be able to explain it better.