What's the effect of nmake on a downloaded Qt package?

[T_h_e_o]

I recently downloaded Qt 4.8.2 . I see that the bin directory contains all DLL's and the lib directory contains the lib files necessary to build.
Now, when I run configure and nmake from a VS2010 command prompt (32 bit, so NOT x64/Win64 !), these DLL's and libs are rebuilt, and they are (slightly) different. Also, the lib directory now contains the DLL's as well.

1) Why are there differences in the delivered files and the rebuilt files (I think it cannot be explained only by included build date/time, there is more difference).
2) What's the use of having the DLL's (the same!) in both the bin and lib directories?

thanks

[ChrisW67]

1. You have potentially used a slightly different compiler and linked to slightly different side-by-side assemblies with different options in force.

2. UNIX-like (sane) platforms separate dynamic libraries from executables and have a separate search path, Windows does not. Windows dev environments typically leave static and link libraries in a separate directory. The build scripts build all the libraries in "libs" and, on Windows, copies the resulting DLLs in to bin so they are found in the PATH at run-time.

[T_h_e_o]

Thanks, Chris.

Now if I want to use those DLL's, should I better use the pre-delivered ones or the ones built myself?
I'm targeting an as broad as possible range of OS versions (from XP to the upcoming Win8), so I suppose that would be the pre-delivered ones as the guys at Qt probably better know what they are doing than I do?

[ChrisW67]

If you have no reason to rebuild Qt then you may as well use the shipped binaries. The only requirement is that the application and the libraries are built using the same brand of compiler: Microsoft, GNU GCC (MingW), ICC etc.