Hello and welcome to the forum.
It seems that you've discovered one more bug in the Monarch v9 xls export driver. It incorrectly sets the number mask in the Comma style to General.
You have a couple of options:
1) Don't the use Monarch v9 Excel export option. Under Options, Export and Clipboard, change from Excel 97+ (Monarch V9) to Excel 97+ (Monarch V8). It doesn't suffer from this glitch, but you won't be able to use the nice features that do work properly, like auto-outlining and auto filtering. Or,
2) Write yourself a little macro to change the number format of the Comma style. Store the macro in your PERSONAL.XLS file, and assign it to a new toolbar icon. Then you can export with the V9 Excel driver, and with one click your comma format will be what you want.
Here's the code I created with the macro recorder to re-define the Comma style that Monarch created for the exported workbook.
.IncludeNumber = True
.IncludeFont = True
.IncludeAlignment = True
.IncludeBorder = True
.IncludePatterns = True
.IncludeProtection = True
ActiveWorkbook.Styles("Comma").NumberFormat = "#,##0.00_);(#,##0.00)"
Both options have their advantages and disadvantages, but there most definitely is an available solution.