Hi Kelley,
Sorry for the delay in responding.
I am able to see the issue. But this is not an XlSIO issue. It is the default behaviour of Excel. At first you added one style and apply for a particular cell. Again you apply the same style to another cell with some modification it also affect the previous formatting. Because the styles formatting are changed. Please refer the video clip:
http://websamples.syncfusion.com/samples/Excelstyle.ZipPlease try to check in Ms Excel and let me know if you have any other concerns.
Regards,
G.Yavana