Attachment not being sent via iCloud Email

Earlier today, I went to email a PDF file (1.6 MB) from iBooks on my iPhone. When the email popped up with the attachment for me to send, the iCloud email was indicated as the sending account. This is what I expected since the iCloud account is set up as my default account. However, when I actually checked my mail application, the email was sent from my gmail account instead. Any ideas why the gmail account was used instead of my iCloud account?