Why did my iPhone 4S go into recovery mode instead of downloading and installing the update after I pressed the install now button in my settings?

There was a new update and through the settings, I went to install it. After I pressed install now, my phone went into recovery mode without warning that there would be no chance to back it up. It isn't a bad thing per se as I did update it just a week ago but the phone itself didnt even try to update. It just asked to be plugged into itunes and once I did that, the update started downloading. So the fact that the button I pressed did not do what it suggested it would do. Quite the opposite really. When something tells me it will download and install a "useful update", but in turn locks me out of my phone until I delete everything off said phone, it makes me more than a little mad.