cat on win32

Hello,
I found doing:
cat -B filename.* > result.bin
will give a file little bigger than sum of bytes of single files.

If I do, instead:
copy /B filename.aa + filename.ab /B + ... + filename.ap /B result2.bin

I have a file that is exactly the result file.

Maybe something wrong with cat compiled for win32?
My file was 81.340.416 bytes, splitted with: split -b 5m

 >cat --version
 >cat (GNU textutils) 2.0
 >Written by Torbjorn Granlund and Richard M. Stallman.
 >
 >Copyright (C) 1999 Free Software Foundation, Inc.
 >This is free software; see the source for copying conditions.  There is NO
 >warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.