Hi Jesse,
Yes, there is a single srpm that will generate binary rpms for each arch. However not every arch gets every package, and some packages are specific to just one arch. s390-utils and the like. This would mean that given the binary packages in a given arch, the source package set could vary between each arch. In the past, the specific source package set was spun into isos for each arch creating slightly different iso sets for each arch and a lot of wasted space. Now all the srpms used for any arch are put in a single directory, and that directory is used to spin a single set of source isos. Much less wasted space and much easier to bypass the sources when you are downloading or mirroring.
Thanks for your explanation. Very clear now :) I have just translated your comment above into Japanese and forwarded to discuss@fedora.jp. Regards, khirano