File modified date changed on copying files

File modified date changed on copying files

In a windows server 2003 R2 when files are copied from one path to another
path i found some files for which modified date is changed to older than
the actual modified date.I found that modified date was about 2 seconds
lesser than actual.Then i compared the files with actual file(binary
compare) but no difference found in content of file. Actually copying a
file should change only created date not modified date. This error is not
repeated after that.But does anyone experienced it or know reason behind
change in modification date on copy?