Viewing Excel file in gmail

I am not an accomplished Excel user; I can get around and do what needs doing in my job. I am wondering if anyone else has had this issue: My accountant and I send an Excel file back and forth each month with a new month of figures added to it. For several months now, however, the accountant has accused me of messing up the formulas, etc., by making unnecessary changes to the document.

I now have noticed that if I merely click on an Excel file as an attachment to a gmail and view it without downloading, the numbers in the Excel file have gone all wonky. However if I actually download the Excel file, then open it to view it, the numbers and formulas are fine, totally unchanged. 

Has this happened to others? It's easy to avoid, but I am curious if it has happened to others and what the reason might be.

Thanks in advance,

Belvel Boyd