Don't get too hung up on the cones edges - remember these are just graphic represantations of the range of most likely tracks. The error keeps expanding into the future so the cones widen

The middle portions of the cones however contain the vast majority of the likely tracks and various entites draw their edge lines at 80 or 90 or 95% probabilities of the track being inside the cone.

This means a track at the edge of the cone is unlikely -- certainly many times less likely than one down the middle --- but there are still track possibilities outside the cone. You are not 100% safe outside the cone.