That may not be an inconsistency at all. Keep in mind also that both are only approximations made during panic mode. The 20 mins might be how long they believe him to have been missing when they first noted his absence and the one hour might include that 20 mins plus 40 mins of searching time before the 911 call was made. Makes sense, to me.
I double-checked the interview and they said they were gone for only 10 minutes. Deorr said they searched for 20 minutes before calling 911, which adds up to 30 minutes. On the 911 call Jessica said he had been missing an hour. So, it is indeed an inconsistency even though it may be an "honest" inconsistency (guesstimating times).