Why is the 64-bit MSBuild loading 32-bit extensions?

I'm attempting to build a project using MSBuild (v4.0) on a 64-bit machine. For some reason, MSBuild is attempting to load a 32-bit extension, and I cannot figure out why. I've reduced the problem to the smallest set in order to demonstrate the issue.

Using the following MSBuild project file:

<Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003" ToolsVersion="4.0">
    <Target Name="test">
        <Message Text="bin path: $(MSBuildBinPath)" />
        <Message Text="extensions path: $(MSBuildExtensionsPath)" />
        <Message Text="extensions path (x86): $(MSBuildExtensionsPath32)" />
        <Message Text="extensions path (x64): $(MSBuildExtensionsPath64)" />
    </Target>
</Project>

I get this output:

Microsoft (R) Build Engine Version 4.0.30319.1
[Microsoft .NET Framework, Version 4.0.30319.1]
Copyright (C) Microsoft Corporation 2007. All rights reserved.

Build started 8/27/2010 9:56:35 AM.
Project "D:\5\test.proj" on node 1 (default targets).
test:
  bin path: C:\Windows\Microsoft.NET\Framework64\v4.0.30319
  extensions path: C:\Program Files (x86)\MSBuild
  extensions path (x86): C:\Program Files (x86)\MSBuild
  extensions path (x64): C:\Program Files\MSBuild
Done Building Project "D:\5\test.proj" (default targets).


Build succeeded.
    0 Warning(s)
    0 Error(s)

Time Elapsed 00:00:00.03

MSBuild obviously knows about the 32bit and 64bit extensions path, and from the binary path it seems clear that I'm running the 64-bit MSBuild.exe, but for some reason it believes that extensions should be loaded from Program Files (x86) instead of Program Files. This is causing me trouble, as I have an extension that I need loaded, that MUST be loaded correctly in a 32bit/64bit process, and it will not load (MSBuild is attempting to load the 32bit version in a 64bit process).

Why?