I send over a thousand messages last month. MyATT shows I've only sent 63. Those 63 were sent as SMS hence why they are "billed" as SMS. THe rest were all iMessages which travel over data / Apple's iMessage servers and not thru the same path as SMS does.
This is why my iPad can use my cell phone number for iMessages, as can my Macbook Pro. Because it's attached to my iMessage acct/ Apple ID and is not inherently related to SMS.