Obviously we can't say for sure what they are thinking and there could be various reasons for this:
- Simple mistake
A breakdown in communication internally has caused the posting to get relisted despite it being filled. Or the instructions to re-list were given before you were hired.
- Playing it safe
You haven't actually started yet so they could be hedging their bets in case something goes awry between now and you starting the job, you stated that they were checking various things on your resume and also that you were relocating. Both of these things add an element of uncertainty for them so having the relisted posting gives them an easy plan B if it goes wrong.
- Multiple positions
Is it possible that they actually have multiple openings for that role and haven't filled them all yet?
- They are planning to ditch you
This is the worst case scenario but one that I feel is also perhaps the least likely.
On the whole I wouldn't overly worry about it - for peace of mind you could always get in touch with them and just confirm that everything is still progressing as expected from their side. This wouldn't be unusual given you are relocating etc and you don't even have to mention the listing if you'd prefer not to.