Design is about empathy.
When I design, I am placing myself in the viewer/users shoes and thinking about what it is they are trying to do/understand. The subtile differences may not be critical when viewed at first glance but the designer thought it affected the experience enough that he/she thought it needed to be slightly different.
Sometimes this is just insanity. Yes, I admit it is a bit crazy to spend 30 minutes on something that no one will really look at twice and I'm completely guilty of it. And as a designer, sometimes we can't help ourselves... because "it just doesn't look right." I can't explain what "right" is but I'll know it when I see it... so I'll experiment with a numer of different approaches.
Sometimes however, it's a genuine care for the experience and the cumulative effect of sweating the small things showcase a level of polish and care that Apple is/was famous for. You just have to decide whether it matters to you (as a company) or not.
Some of the design choices made for the two versions seem like just an evolution in the style for the app that weren't back-ported to the older version. For example, the different colors of orange for the button, gradient shading, etc. Other changes are adjustments that, from his eyes, seemed necessary to make the experience better/acceptable on the 2x version, such as the sizing of the Search button, border bevel, etc.