Your build server is using the FxCop binaries from Visual Studio 2012, not those of 2013. Each time a new version of Visual Studio is released there have been some updates to the rules to fix bugs and there have been a few rules that got added. There's also a difference in certain code generation scenario's between .NET SDK versions.
Given this knowledge, I am not surprised that there are different findings from the different versions of the tools. How to fix it you might ask? Install Visual Studio 2013 on the build server and make sure your "Run MSBuild for Projects" activity in team build points to the correct toolsdirectory (the MsBuild one that ships with Visual Studio 2013).
Or: Make sure you're using TFS 2013 and Team Build 2013, which should figure this out by itself if both versions of Visual Studio are installed.