Why does XCOPY /D not always work

I am using XCOPY to backup files from my c: drive to a file server. I only want to copy files that are newer on my source drive than on my destination drive. I’m using the /D option. There are 6 files in my source folder. Everytime I execute the command, 2 of the 6 files are copied even though the modified date and time on the source and destination folder are the same. Why is this happening?

xcopy “C:\techword save\*.*” q:\AllProj\special\ /d > q:\AllProj\xcopypwd.log

The source file has the following file types:
.bak
.exe
.dat
.hlp
.txt
.cnt

The two files that get copied each time are the .bak and .dat files.