That message refers to your total messages Inbox + Sent Items. I'm not sure why it only shows in your Sent Items folder. Sent messages are only saved in the Sent Items folder if you select the option "Save a copy to my Sent Items folder" before sending the message. This has been reported before but it is working properly, just make sure to click that option if you want a copy. You can also make this happen by default by changing your user settings.