If they are true iMessages (iPhone to iPhone) they are typically sent over the data network and are NOT counted as text messages. Therefor they are not shown in the text messaging usage details
Are you asking why iMessages are sometimes count as text?
Sometimes when the iMessage goes through it doesent always go through as iMessage although it says it will be. I have had a lot of those. If you don't have a text plan and your charge you can get a refund if you prove it was an iPhone to iPhone. I hate how that happens. It can be a true iMessage spiral with still be charged. I had blue all the way and I was still charged for 2. I don't know what you mean by a real iMessage. Sometimes that's a Verizon saying a real iMessage they didn't even know there was a don't send SMS if iMessage doesent work. It still tries with that block. All they can say is do a text block but I will pay for texts other then iMessage. I shouldn't have to block full text to have iMessage to be right fix the issue not do a work around. Verizon should know this is an issue.