Backfill operation runs but summary index data is missing a lot of entries

I'm having trouble with backfilling a scheduled search that populates the summary index. Here is the basic search:

index=cdnmanager | bucket _time span=10m | stats sum(Bytes) as NumBytes, sum(Elapsed_Time) as ElapsedTime by Base_Content,_time

When I run this search on a small dataset I get the expected results – there are 10-minute wide timestamps, and one entry for each timestamp. A small sample follows:

_time Base_Content NumBytes ElapsedTime 
1   1/24/11 8:00:00.000 PM  a_e       6636911       2116055
2   1/24/11 8:00:00.000 PM  ae        8554          368082
3   1/24/11 8:00:00.000 PM  amc       16695         10710
4   1/24/11 8:00:00.000 PM  cnn       8666          8111
5   1/24/11 8:00:00.000 PM  comedy    7530079       1285838

This search is scheduled as follows:

action.summary_index = 1
action.summary_index.report = bandwidth_by_service
cron_schedule = 0 * * * *
dispatch.earliest_time = -70m@m
dispatch.latest_time = -10m@m

The idea here is that I want to roll up some statistics into 10-minute buckets, and I want the job to run every hour, and look back in a window slid backwards in time by 10 minutes, in order to allow data to make its way through the LWF into the Indexer.

The problem is that when I run the backfill, I am only seeing a very limited number of entries in the summary index, and the few entries that exist are all of the form hh:50:00 which means they are all for the 10-minute window spanning 50-59 minutes after the hour. There is definitely a full range of entries covering the entire hour, so it's not a data problem. Can you figure out why this is happening and tell me how to correct the search and/or the schedule configuration data? I think I must have something configured incorrectly, though damned if I can figure out what it is. Thanks!