That's unfortunately how Excel works.
If you open a legacy Excel spreadsheet file, Excel updates the file timestamp to the current time (making WinSCP believe you have saved the file). If you then close the file without changing it, Excel restores the original timestamp.

Hi Martin, any thoughts on this? I want to stop WinSCP from saving XLS files that I open, but don't change. For some reason, it seems like WinSCP thinks my XLS file changed...

So when I open and close an XLS file from WinSCP (without making any changes to the file's contents) the file automatically gets uploaded and saved on my SFTP. The "Changed" date stamp gets updated and ownership changes if I wasn't previously the owner. However, if I do the same to an XLSX file, the file does not automatically upload, which makes sense because I didn't make any changes to the file's contents. Any idea why this is happening to my XLS file? I don't want a file to update just because I open and closed it.

Thanks!