Well, I can’t say that I know the exact reason for this, you need someone from the SL team to categorically state the reason, and I did not see any blogs from them.
However, I strongly suspect that this is due to the fact that there is another Silverlight plugin, that is, when the user installs the Silverlight 4 plugin, it replaces the existing one. The Silverlight 4 plugin will launch not only Silverlight 4 applications, but also Silverlight 3 and 2 applications. As a result, Silverlight 2 applications will look for libraries 2.0.5.0 and avoid certain problems if later versions use the same version number.
Of course, I could leave and hope that if I am an MS teacher, go ahead and shoot it, during which we will give a real answer.