Perhaps Apple chooses not to publish themselves as the assignee because they don't wish to make obvious the directions in which they are moving. However, this raises two questions. First, is the Apple assignment recorded in the patent office assignment database? If so, the data can easily be mined there. Second, if Apple is not listed on the patent or in the database, how does UBM know they are Apple patents?
Apple is successful not just because of its phone but it successfully built ecosystem by integrating iTunes. People don't look at patent portfolio when they want to buy a phone, they just look how user friendly the phone is.
Apple has never been the inventor of new technology. They have been great - probably the best company ever to integrate ideas from others and package it into a seamless easy to use manner.
Apple did not invent the mouse of the graphical user interface but they brought it out of a xerox lab and into the mainstream. They did not invent MP3 players but they found a way to make the mp3 players cool and easy to use. They did not invent the touch-screen but they found a way integrate it into a phone.
Not trying to belittle the design innovation at Apple but patents are issued for new non-obvious technologies. Everything in apple products are made by someone else - Apple just does a great job at integrating this technology.
I never see Apple as innovative in the usual IBM, Xerox ways. I know them as making better out of what exists. So, you do not have to worry if Apple has a patent. Most of their patents are like art works in colleges.