Windows 10 quality updates explained and the end of delta updates

Hello Bryan,
It sounds like you are asking what is stored on the windows update service and available for clients to download.  Generating differentials takes time and space on the service, so we generate the ones that will be most likely to be used by the highest number of users.  We did some data analysis and identified that over 90% of users are within N-5 of being current (with the majority being at N-1).  So we generate differential baselines for N-1, 2, 3, 4 and 5.  And we also generate a differential for RTM.  
When a device goes to download an update it chats with the service to let it know what version of a file it has, and then the service determines which differential would be the best fit.
Technically we could generate differentials for every possible baseline permutation, but that would take more processing time and more storage space.  And as you see from other people on the thread, it also impacts the disk size and download of people running WSUS who want to enable express locally (since the same files we store for express on Windows Update are also sent to WSUS).  So we try to make the best tradeoff across both experiences.

Mike