Windows 2003 Snapshots schedule

Hello, in a Windows 2003 server x32, Standard, in a 300GB disk, i´ve made a
60 GB partition for the operating system,©, and left the rest for Data. I
want to make snapshots of the data partition, I´ve scheduled the start at 7
am , then set a copy every 6 hours through 13 hours.According to this, it
should make a copy at 7 , 13 , 19, right?
Well, it does them , but also make one at 12.
Can anyone knows what I´m doing wrong?
It´s not a matter of live or dead, but I´m curious why it happens.
Thanks for any inputs you may give me in advance.
Greetings