Board Thread:General Discussion/@comment-24491127-20160222124014/@comment-27401333-20160223131021

True, altough that's just the Neutral ending. Frisk still has time in the Underground, as seen by the Pacifist ending. So it's possible they never left the Underground, and that the Neutral ending may not be considered "true" or "canon" to some. Many people tend to see the Pacifist ending as "canon". It may be that the Neutral ending is more of a what-if scenario.