Sid is not a rolling release. This is a common misconception.
In preparation for a new stable release, it goes through what is effectively a package freeze. Maintainers are discouraged from updating packages in Sid in order to focus on the new release. So almost no new updates are uploaded to the repositories until Stable is finally out.
That's not exactly a hard rule, just stuff maintainers do because it is far easier to still have normal flow of unstable -> testing for the freeze period, and keeping 2 ("actual unstable" + "unstable on the way to the new release") would be a big burden for maintainers.
In preparation for a new stable release, it goes through what is effectively a package freeze. Maintainers are discouraged from updating packages in Sid in order to focus on the new release. So almost no new updates are uploaded to the repositories until Stable is finally out.