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Well it is a choice. They COULD have made the update system as a primary version and then a series of deltas. The flipside of that would be that any reinstalls or new installs would require to first load the old full and then maybe scores of updates, installing all in the same partition, and this could be slow. And it would add some extra complexity in generating and applying the deltas. It is much simpler to just have a source that builds a total, and then distribute that, despite the extra traffic, and slower installs than the delta could have given, if it started by making a copy of the run

The point is that the updates are not incremental. The weekly OTAs each time contain the full ROM, even if changes relative to the previous one are minor or non-existing. The idea behind the a/b partitioning scheme is to keep the previous version of the system intact for backup, while flashing a completely new version of the OS to the respective other slot with every update. This is to make sure one of the slots always contains a working OS, even in case of a bad flash or faulty download.