Full - Differential - Incremental (all the same size)

Have supposedly the most current build for server 2012 (43800) and I believe this topic has been discussed many times but I still have yet to find an answer. I just backed up a rarely used 1gb directory using "Backup Now / Full". After it was done I backed up the same directory using the "Differential" method and then backed up the same directory using the 'Incremental' method. This all took place within 15 min. Each of the backup files were exactly the same in size!  Why??? I did this because my initial "Differential" plan that followed a full backup of my server did the exact same thing. There is no defrag software running.  I really hope I'm missing something obvious.

Thanks for the help