Grtz, Crushroom
How long were your scavengers out?
If they were out 8 days then it'll take them 8 days to return to your vault.
I haven't read anything that says it should only take 1/2 the time for a explorer to return to the vault.
Would you happen to have a link that does state that supposed fact? Or, are you just stating your opinion on the matter?
Have you changed the date/time settings on your device?