Are you saying one updates OS automatically? Explain that.
yes exactly. One phone, whenever an ios update is available, doesn't prompt me to do anything. I won't know there's an update available unless I see the badge icon on the Settings app. And then when I go to settings-general-software update, it tells me I need to be connected to both an AC and Wi-Fi source to download the update. so after I connect the phone to Wi-Fi and AC I first have to click 'agree' after tapping 'download'. and then after it downloads I have to click 'agree' after manually tapping 'install'. and that's pretty much how all iOS updates have acted on any iPhone i owned. this is normal typical behavior as far as I'm concerned.
The other identical phone apparently downloads the software in the background when the phone is neither on Wi-Fi nor AC power. so this morning, as an example, I had that phone in my hand and was texting somebody and all of a sudden a dialog box popped up and said 'iOS update 10.2.1 is ready to install'. and it asked me to either tap 'install now' or 'remind me later'. because this has happened with the last two or three iOS updates since I bought the phone in September, I was used to it, so I just tapped 'install now'. further, what's equally odd is the behavior when I click install: the the screen goes black, then the progress bar for installing appears with the white screen, then a second later goes black again, then the white screen appears again, goes black, and then comes back again, and then the install proceeds typically after that. but every install I've done before on my other iPhones goes differently: normally, when the install initiates, the screen goes dark and then the white progress screen comes up just one time and stays like that until the phone reboots. So this is another thing that's just totally bizarre.
a lot of detail, but that's pretty much it.