Different formatting every time you open report in BI Publisher

Summary

What makes the formatting done in .rtf template change every time you open report in BI Publisher?

Content

Hi. I am creating reports in BI Publisher using .rtf templates and noticing that often when I upload the document to BI Publisher, formatting is wrong. Wrong in a way that distance of header from the top margin is different than I expected. If I upload the same document again without making any changes, suddenly the formatting is as desired. 
Currently I am using

<?call-template:header?>

to fetch a header image. Wondering why viewing reports at different times shows different formatting. What is it that I am doing wrong? I would really appreciate some help.