I appreciate there's a nerdy "IT MUST BE CORRECT" aspect to this but Apple are making billions - why would they bother doing work and creating potential support problems for themselves to gain maybe a fraction of a percent more?
You're welcome to start listing the actual umpteen problems that a billion dollar company would incur rather than just talking out of your ass about "Apple's billions" to a bunch of people, myself included, who have most likely implemented similar personalization systems at countless other sites using a lot less human, financial, and processing capital than Apple has. Apple "wastes" a megabyte for a picture of a wave on their current home page. Any change here is likely to move the needle in a positive direction more than the increased sales from having a massive picture of a wave on their home page.
But even doing this server side isn't that bad, calculating the difference between two sets, the top 100 apps and the apps a user owns, iterating with more if the size of the result is less than 10, is a lot less work on a per-user per-request basis than a lot of sites do to customize their experience.
Of course, Apple most likely doesn't want anyone to know which app is the 11th or something, which means a lot of server side processing to customize the exact list of 10 sent to the client. Sites like Facebook already do this kind of stuff for calculating the timeline for hundreds of millions of accounts.