Events will be timestamped so perhaps subsequent searches are finding events in the same time frame which weren't present when the summary index was created. Have a look at the _indextime field for the events to see if there is a spread which would account for this. Also, have a look to see if your events have been duplicated in your subsequent searches.
Other things you could check is whether the data in your summary index is correct (for the event which were present at the time they were added to the summary index.
