When using the feature "Export to Excel", only the first 300 rows are returned.
This limit can't be changed or disabled.
The intention of the option for 'Export to Excel' or 'Export to PowerPoint' is to provide a nicely formatted output. Therefore, a restriction on the number of rows to be exported into this format is implemented.
There are currently several Ideas in our CA Community that are "Under review" by Product Management regarding changes to be done to this feature.
Use "Export to Excel (Data Only)" instead, as it does not have a 300 row limit. This option allows the user to export the numerical data into a spreadsheet and then it can be formatted as desired.