Apple Vice President of Software Development Craig Federighi explained why automatic iOS updates can take up to several days or weeks, while manual updates can be installed immediately following release.
iPhone co-owner Mateusz Buda asked Craig regarding it directly and got the answer:
Hi Mateusz,
We are gradually rolling out new iOS updates, first making them available to those who explicitly search for them in settings, and then rolling out to devices with automatic updates enabled 1-4 weeks later (following receiving update feedback).
Hope this helps!
– Craig
