Releases of the web APIs are not tightly bound to releases of ArcGIS Server. So while some releases may be timed closely with those of ArcGIS Server (to allow you to leverage new server functionality in your web API apps), they are not necessarily so. This is especially the case with service packs, where any new functionality in the server, if present at all, is usually very limited.
As I'm sure you know, 2.1 is currently in beta. Final will be released soon.