Why I fly when in Route 26-27 Pokemon HeartGold?

You can't actually fly in Pokemon HeartGold and SoulSilver on Route 26-27. You can fly to specific locations, but only after obtaining the HM Fly from the Fly HM tutor located in Goldenrod City.

Here's why you might feel like you can fly:

* Bike: You can use the Bicycle to move faster on the route. This might feel like you're flying since you're moving at a faster speed.

* Route Length: Route 26-27 is a fairly long route, which could lead to a sense of traveling long distances, even if you're only using the bike.

* Visual Cues: The route is designed to give a sense of open space with the vast ocean and surrounding areas. This might also contribute to a feeling of flying.

If you're trying to fly in the game, you'll need to learn the HM Fly and then use it to fly between various locations you've previously visited.