Why version of SBT is playing role in name of fully qualified dependency?

Why version of SBT is playing role in name of fully qualified dependency?

Why version of SBT is playing role in name of fully qualified dependency ?
I thought it only depends on version of Scala.
Here is example.
build.sbt has scalaVersion set to 2.10.2
build.properties has sbt.version set to 0.12.1
plugins.sbt has plugin dependency "com.github.siasia" %% "xsbt-web-plugin"
% "0.12.0-0.2.11.1"
If I build it I see following in the log:
[info] Resolving com.github.siasia#xsbt-web-plugin_2.9.2;0.12.0-0.2.11.1 ...
But if I change sbt.version set to 0.13.0-RC5 SBT fails to find this
plugin and says in the log
[info] Resolving com.github.siasia#xsbt-web-plugin_2.10;0.12.0-0.2.11.1 ...
Why did it change scala version if it remains 2.10.2 in my config ? How to
fix it ?