Questions about iOS Notifications?

I've noticed something more recently, and I'm not sure if it's always been this way or if I've just become cognizant of it, but many times when dealing with notifications from apps, older ones don't clear--even when the newer ones have been dealt with.


In this case, I dealt with the notification that said "now" first. I replied to the message, and yet when I swipe down Notification Center, there it is along with two other notifications that I've obviously read if I went into the app to deal with the latest one. I've noticed that Instagram does this as well, but I've seen notifications roll away before my eyes in Notification Center before if I've dealt with a later one on other apps. Why does this happen? Is it sloppy code on Google's part, or sloppy cleanup on Apple's part?