Hyperlink Pointing To InfoPath Form Launches Excel for Some Users Instead of InfoPath in a Browser

So....here's the deal...

• We have a form library
• I have a workflow that sends an email with a link to the form to an end user
• Sometimes that email is forwarded to other end users
• We all have edit access to the form
• Some users who click on the link get taken to a browser window showing the form - GREAT!
• Other users who click on the link end up launching Excel - NOT great!

Does anyone have any idea why some users get the browser version and some get booted to Excel?