I suspect your confusion comes from the fact that the vSphere client shows the Average, Minimum and Maximum over the past month for the cpu.usage.average counter. Besides that there also exist counters, but not available in statistics level 1, that are called cpu.usage.maximum and cpu.usage.minimum.
To simulate what you see in the vSphere client you can do
$entity=Get-VM-NameMyVM
$start= (Get-Date).AddMonths(-1)$counter="cpu.usage.average"
Get-Stat-Entity$entity-Stat$counter-Start$start|
Group-Object-Property {$_.Entity.Name} |%{ $stats=$_.Group|Measure-Object-PropertyValue-Average-Maximum-Minimum
New-ObjectPSObject-Property@{ VM=$_.Name
CPUAvg=$stats.Average
CPUMin=$stats.Minimum
CPUMax=$stats.Maximum
} }