以 nokogir 为例，最新的版本是 1.8.0，但是执行 gem update 无法更新到最新版本。执行 gem install 就可以安装最新版本。

[5] pry(main)> Gem::SpecFetcher.fetcher.search_for_dependency Gem::Dependency.new 'nokogiri'
=> [[[#<Gem::NameTuple nokogiri, 1.7.1, ruby>, [Remote: https://gems.ruby-china.org API URI: https://gems.ruby-china.org]],
  [#<Gem::NameTuple nokogiri, 1.8.0, ruby>, [Remote: https://rubygems.org/ API URI: https://api.rubygems.org/]]],
 [#<Gem::PlatformMismatch:0x007f8ed9091720
   @name="nokogiri",
   @platforms=["x86-mswin32-60", "x86-mingw32", "x64-mingw32", "java", "x86-mswin32", "x64-mingw32", "x86-mswin32-60", "x86-mswin32", "java", "x86-mingw32"],
   @version=Gem::Version.new("1.6.1")>]]

同样的接口 rubygems.org 返回的是最新版本，gems.ruby-china.org 要旧一些。