Are you doing a join of two tables ? Or is it a single table query as your example shows ?
In the former case, it may be that in the first 7 minutes, it had selected x number of records while still performing joins ... It wouldn't have been able to find a matching row for the next 13 mins .. (Poor performance, but theoretically possible)
In the latter case, it is not a 'normal' thing
Cheers
Sathyaram