Facebook Message sent to person online. Imediately their status goes to last active. Any idea why?

I have a Facebook friend I message back and forth with. Many times they will be shown as online and as soon as I hit send their status immediately changes to active x minutes ago. Before I send I will go back and forth a couple times to make sure the online is accurate. They then become online a couple minutes later. This happens a lot. I know they use an Iphone and leave Messenger open alot when they aren't actually using it. It seems like my message kicks them offline.