Why does my iPad require me to tap 'OK' for a 6 digit PIN, but my iPhone does not? I'm using a 6 digit PIN on both my iPad and my iPhone.

Both are running iOS 9.0.1. 

My iPhone takes the six-digit PIN and opens to the home screen. 

My iPad takes the six-digit PIN and then waits for me to touch OK before I can go to the home screen. 

I have the iPhone 6S and the 3rd Gen. iPad 

So,why does my iPad require me to tap 'OK' for a 6 digit PIN, but my iPhone does not?
On your iPhone, you have a 6 digit code, replacing the prior 4 digit code.

On your iPad, you have an alphanumeric password, which just happens to consist of just 6 digits. Alphanumeric passwords on iOS are just like other passwords in that you have to confirm when you have finished typing.