about compression rate of BACKEXEC 2012

Hello everybody

Today, I have question about compression rate of Sym BACKUPEXEC 2012. My comapany used this version for backup operator daily( backup VirtualMachine and Virual Disk ) and set compression type is " software" . Example I need backup total 12 VMs but when I read backup job then

• backup VM 1 used compression rate is : 1.9 :1
• backup VM 2 used compression rate is : 1.5 :1
• backup VM 3 used compression rate is : 1.8 :1

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That's mean doesn't used same compression rate.

I don't known why ? Please see explain to me

Thanks/phamduyus