Why is the binary output not equal when compiling again?

Question

I'm using a build script to compile several C# projects. The binary output is copied to a result folder, overwriting the previous version of the files, and then added/committed to subversion.

I noticed that the binary output of the compilation are different even when there was no change to the source or environment at all. How is this possible? Isn't the binary result supposed to be exactly equal for the same input?

I'm not intentionally using any kind of special timestamps anywhere, but does the compiler (Microsoft, the one included in .NET 4.0) possibly add timestamps itself?

The reason I'm asking is I'm committing the output to subversion, and due to the way our build server works the checked in changes trigger a rebuild, causing the once again modified binary files to be checked in in a circle.