Report runs faster in the background after selecting multiple formats

[ClintSAP]

I've got a very strange situation that I hope someone has a explanation. I've got several people who have built reports and the reports are taking for ever to execute and hitting 3600 second time limits. They have done everything they can to make sure the reports are using efficient queries. Some times these reports are only pulling a very small data set and they still take for ever when trying to run the report in the background to Excel. Big HOWEVER, someone decided to test running the report in the background using both Excel and Excel Data and miraculously the report ran very fast. They have been able to duplicate this with many other reports. So my question is, why would selecting multiple output formats cause the report to run faster where it was hardly running at all with just Excel?