Why is Bundler is vendoring ruby?

I have a bunch of Rails3 projects under active development, and Bundler normally behaves just fine. On one project, whenever I run bundle install, it vendorizes ruby its self -- binaries and all. Why? How 开发者_运维问答can I stop this?

Project in question:

...
/vendor
  /ruby
    /1.9.1
      /bin
      ...
  /plugins

Check .bundle/config at the root of your project. it is probably configured to do this.