I have written before (several times in fact) about the challenges of exporting to Excel when you have subreports. It is difficult to avoid merge cells unless you get the alignment just right. Recently one of my readers told me that he exports to TTX format in this situation and finds it works better. I did a quick test using a report with a subreport on GF1. The subreport was just a bit below the other fields. When exported to:
1) Excel there were extra merge rows between the data rows
2) Excel (data only) the subreport was on a separate row
3) Tab Separated (TTX) the file would open cleanly in Excel
I am sure there are other variables but if you need to export to Excel format and there is a subreport involved, this is one more option to try.