On export to Excel the spreadsheet is creating an additional Row to accommodate the Heading Data that does not line up correctly, similar to an additional Column being inserted when the columns do not line up vertically:
On Expanding the rows you can see this more clearly, with the rows highlighted in blue:
To troubleshoot, click the Group Header section and review:
Properties > Layout > Size > Height
Click on the cells in the heading and check that the Size Heights agree to the Group Header value for example:
A single row height = Group Header height
Multiple rows height summed = Group Header height
Example 3 rows of 88 = Group Header of 264
You may need to check all of the individual cells in the header to make sure they are all giving the correct values.
If they are still not lining up:
Properties > Text > Summary > Can Grow
Make sure the Can Grow is not ticked as that may be overriding the control of the row height on report generation.