If the routing deviates too far from the direct route, the result is a “stepped” routing. The routing ends up on the edge of the isochrone. To avoid this, increase the sweep angle in the routing parameters. The default setting is 220°. If your device is fast enough, set it to 360° and you'll never have to wonder again.
If you haven't taken the Premium option, add an intermediate target. You can place it on the stepped routing, towards its middle.
With a search angle of 220°, you've got stairs.
With a 360° search angle, the stairs have disappeared.
