It's pretty well known from Undertale that different timelines exist. Deltarune may simply be a different timeline but with the same characters.
It's also worth mentioning however that if you fight Sans in Undertale on the genocide route, he hints that he knows these different timelines. And after you finish Deltarune on Pacifist, you meet Sans again and he acts like he already knows you.
Couple of other fun facts for the people who may not know, if you rearrange the letters for Ralsei, it spells Asriel(Ralsei is Asriel though) and if you rearrange the letters to Deltarune, it spells out Undertale.