I assume you have reduce data set in the document properties. If that is the case, then what you are seeing is that the section access settings limit what you can see.
If you are allowed to perform a reload, however, you will be able to see all the data as the data reduction is only applied when the document is opened, and not after a reload. (If you can perform the reload, then you have at least read access to the data sources, so the data is not secure anyway).
If data security is important, then user reloads should not be permitted and the source data should not be visible to the users either.
Logic will get you from a to b. Imagination will take you everywhere. - A Einstein
It looks like the Excel data is associated in some way with the fields in the section access table. If you post a sample application with some source (qvd or xls) that illustrates the problem, then you will get more detailed and accurate assistance.
AFAIK your post contains nothing out of the ordinary. Section access & data reduction manage access to specific data values-rows-columns at document open-time only. The very first action that QV performs when opening a document with SA and data reduction is to verify who you are, look up your ID in the SectionAccess-table and throw out all data that is not associated in any way with your ID. If you keep your document open, this process will never be repeated.
Meaning that if you - as an ACCESS=USER person - are allowed to reload the data in that open document, you will suddenly get access to everything that the script loads from your data sources, including all data without associations (apparently your Excel fields don't have any associations). If you then save-close-reopen your document, you will again fall back to SA and data reduction or you may even get Access Denied.
That's why giving ACCESS=USER document users permission to reload a document is not a good idea.