It's just marketing. People latch onto the 2 years + 1 year of security updates, but drop the second half of the sentence. Google could have just said 3 years of updates and just had an asterisk to describe 2 + 1 years.

The last supported OS version on a given iPhone version runs like a pig. iOS 9 is noticably slow on an iPhone 4s, and iOS 10 on an iphone 5-no-'s' is no exception, so I'm not sure how much that fifth year of support really counts for.